Have you ever climbed on the bus without knowing quite when to get off?
Perhaps you hope to get engrossed in your book or phone, or you'd like to catch a few Zs along the way. Perhaps you're going to a new stop, and you're not sure just when to pull the cord so you'll be past the previous stop without shooting past your intended destination.
This app should, at a minimum, notify you just as the bus or train passes the stop just before yours. It should probably have a setting for some additional advance notice, in case you need time to gather your belongings or collect your bike.