Even someone whose finances are a mess can teach you something valuable about money management

People are often more willing to talk about their sex lives than their money habits.

So when money diaries first started appearing online, they were met with instant excitement. Because who doesn't want to read details of someone's personal financial life?

The website Refinery29 was among the first to launch personal financial accounts in 2016, and they quickly became a hate-read for some people. In the beginning, a number of the writers didn't seem to have many financial constraints about spending money.

Others soon joined in, including The Billfold and Man Repeller. Robo-advisor Wealthsimple also published money diaries with higher-profile people such as Kim Kardashian. Even The New York Times occasionally runs a money diary.

As a measure of how Americans view their spending, 52% of respondents said they'd be willing to give up takeout and food delivery in the Invest in You Spending Survey. The national survey of 2,800 Americans was conducted June 17–20 by CNBC Invest in You + Acorns in partnership with SurveyMonkey.

Alicia McElhaney, founder of She Spends, a personal finance website aimed at women, says online financial diaries offer several valuable money takeways.

Even if details are sparse, a careful reader can still pick up clues about someone's values from what they spend money on.

Spending on specific items or events is mysterious to a lot of people, McElhaney says. "It's good to be open, and it can invite a lot of judgment: especially the weekly thing."

Many diarists tend to be the really high earners and people who get a lot of help from their parents, McElhaney says, or, conversely, people who make a really small amount of money.

What generally sparks the most reader irritation is when people who get financial support from their families are not upfront about it in their diary, McElhaney says. (One writer who did disclose her family's support still managed to infuriate people: "Skipped right to the comments after reading what all her parents paid for PLUS the allowance from the grandparents," one commenter said.)

"Money is a fraught topic, and it's hard for us to talk about it — and talk about it in a healthy way," McElhaney said.

The length of time may also lend some unreality to the diaries. One week doesn't speak to life's natural variability, McElhaney says. "Life happens. Things come up. We're different from week to week, and that's OK."

Debi Patton, 63, says she has little in common with the writers of the money diaries she came across. As a retiree living in Cumberland, Maryland, the $7 cups of coffee and $100 jeans she read about weren't relatable.

At first, Patton says, she read people's spending accounts and found herself judging their financial habits. "I was struck by how few of the people even mentioned any savings at all," Patton said.

Her own spending habits are restrained: She describes herself as really frugal and tracks her finances on Mint. But then her judgments began to soften.

"It sounds like a lot of people are making the same mistakes I made when I was working in my 20s and 30s, making good money and not saving a penny."

As she read on, she found herself thinking more about the similarities between someone else's story and what she is doing in her own life.

In the end, Patton found herself judging her own finances more keenly.

Her grip on her own finances, while restrained, may need some tinkering: She spends more than she'd like on restaurant and takeout food. "I might be letting little bits of money slip away without even realizing it," Patton said. The diaries are fun to read, she says — and could motivate someone to pay more attention.

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SheSpends also runs money diaries, which are more of a snapshot than a diary with comments and details. This allows the money details to stand out more starkly, McElhaney says. "Here's the salary; here's the savings amount."

The money diaries were integrated into She Spends from the site's beginning. "Looking at the other diaries, there were parts that were missing," McElhaney said. "A week in time can be a great way to see how you're spending your money, but it doesn't get into what you value spending your money on or what you don't value."

Also missing from other diaries: charitable giving. "I think that is a really cool part of [She Spends diaries]," McElhaney said. "It makes it feel like almost a mandatory part of your budget, and it should be for a lot of people."

Money diaries can help others. "I think it's helpful to see how different people manage their money," McElhaney said. "Tools and tricks they bring up can be a great way to learn. It's an easy way to talk about money."

Stock options and exchange-traded funds aren't everyone's idea of the best way to talk about money, McElhaney says, and this way, you are still talking about and learning about money in some way. "This gives them an easy way to talk about money," she said. When you start defining goals and dreams, it can be a jumping-off point.

Jessica Byrne, 26, a software engineer in Portland, Oregon, is a constant reader of money diaries. She likes the details of people's spending habits in different jobs, cities and at varying income levels. "Talking about money is so taboo," Byrne said. "When you get to read about how people are actually spending, you get more context in how high-earners spend, how people spend when they're on a budget."

Byrne has her own personal finance blog and wrote her own money diary for Refine My Money.

One thing Byrne noticed about writing a money diary: "I kept wanting to put it off," she said. "No week was ever average. There was always a weird expense, like a medical expense or a plane ticket."

Wanting the diary to be a consistent snapshot, Byrne eventually realized there would always be some unusual expense, so she just dove in.

Next, she was anxious about appearing dull. "So I was influenced [by writing it] to go out on a date with my boyfriend and get pho," she said. "I think we might have skipped it."

Many of her friends noticed how much of her life is subsidized by small freebies — she details drinking free IPA at work and taking advantage of free couples' counseling through her boyfriend's job. "That would normally be $250 to $500 per session," she said.

Byrne learned something interesting while keeping the diary: You track not only money, but time. "You get to see the small patterns of your day," she said. "You may not think you're doing or spending much, but you may find out you're doing more and spending more.

"There's no one right way to spend or save," Byrne said. "Being able to see there isn't a prescribed way to spend money when you're trying to measure yourself."

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Disclosure: NBCUniversal and Comcast Ventures are investors in Acorns.