"When it comes to worry, that function is pretty clear: It draws our attention to the fact that there's something we maybe should be doing or preparing for or preventing, and it gives us the motivation to do something about that."
That can lead to some important choices. If you worry about a car accident, you'll wear your seatbelt. If you worry about skin cancer, you'll wear sunscreen. Worry prompts you to do something that might be inconvenient, but protective.
It also makes you prepare: If you fret about a job interview, you'll spend more time preparing for it. Worry tells you a situation is serious and keeps it prominent in your mind so that you take action.
"If we don't ever worry about our future, we're likely to put ourselves in some significant danger and risk," Sweeny said.
Even when you don't have control over what's about to happen, worry can serve as an emotional buffer. The feeling is so unbelievably unpleasant that, at the very least, anything that happens in the end will feel better in comparison, she noted. Bracing for the worst can actually dull bad news and make good news feel even better.
The conclusion of Sweeny's analysis: Worrying the right amount is better than not worrying at all.
Easier said than done.
We're all wired to worry, though how much we actually do depends on our genes, parents and personal mindset, said Melanie Greenberg, a clinical psychologist in Mill Valley, California, and author of the new book "The Stress-Proof Brain."
She agrees worry can have an upside, but warns that too much of it can lead to problems. About 85 percent of the things people worry about never happen, she said.
Here's how to worry better: