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.