Yo is, from what I naively understand, is a super simple app which you can say "yo" to someone. The analogy, I assume, is that someone so simple can still have a use?

And behold, here is a use for the Yo app...interesting...

The question is whether using Yo is more practical than using a Timer app, which is native to every smartphone released in the past 5 years.

It isn't.

It might be, if you can't remember how many minutes it should be. I have to admit I always have to look it up,

One could also build the same functionality into a Twitter account fairly easily.