In any case: being able to hail a taxi on the street doesn’t mean it can’t be useful to pre book one with an app too. I you want to go to the airport in the morning from a house where few taxis pass, that’s what you need to do. In the 90s that meant calling a phone number but these days you want to use an app, pre pay in the app and so on.
(On a side note, I take buses every day but I have never seen someone signal for one. If I’m at the bus stop it stops, if I’m not - it doesn’t. This varies between cities too I guess)