I'd run the bus lines every 15 seconds if I ran the place, but the point of real-time travel data apps is that they are a very cheap way to add customary functionality to systems which already have GPS information for their fleets. People hate waiting for buses/trains more than they dislike the time spent riding them, especially if weather is bad, and they hate waiting even more if there's uncertainty. If you're reasonably sure that bus is going to come in 8 minutes, the 8 minute wait isn't so bad. It's more frustrating if you don't know when the bus is coming.
Increasing frequency costs a lot of money, money I would of course spend if I were your benevolent transit overlord. Transit apps are cheap.