With modern technology, it's pretty easy to just type some words on your phone and have a computer voice read them. Given that, why do people still speak words using their mouths? What's the advantage?
The answer, of course, is that for most people, speaking is much faster and easier than typing words and having a computer voice read them. If I have something I want to say, I can just say it without having to type it.
Of course, a lot of people have disabilities that make it difficult or impossible to speak, so for those people, assistive devices and software can be very useful.
The same goes for musical instruments. If I'm a skilled pianist, then playing the piano is much faster and easier than entering notes into a computer program and having the program play them. If I have something I want to play, I can just play it without having to enter it into a program.
Granted, most people are not skilled pianists. But playing the piano fluently is a skill that can be learned.