It’s to make sure you show up for your actual trial.
Obviously she committed a crime, so she was jailed, but they have to do all the work to put the case together to prove it at trial. During that time, they don’t want you to skip town and avoid justice, so they put you in jail. But keeping people in jail is expensive, so you can give them a pinky-promise based on your reputation (“personal recognizance”), or something of value that you get back when the trial happens (bail money).
You can still skip bail, you’ll just lose that money, and I’m sure there are additional charges for when they find you.