A lot of things can cause your battery to drain quickly. If you have your screen brightness turned up, for example, or if you're out of range of Wi-Fi or cellular, your battery might drain quicker than normal. It might even die fast if your battery health has deteriorated over time. Some of those things are out of your control, but there are ways to better manage the battery on your iPhone, and Apple makes it easy.
That's the gist of it. It's simple to see the apps that are using up your battery. This is particularly useful if you download a new app and notice that your battery starts draining really quickly. Sometimes bugs might cause this to happen, and you'll know what's causing the problem and to delete the app until it's fixed. Or you might discover that your battery health isn't as good as it once was, and that it's time to get a new battery at the Apple Store.