Have you watched GrootBoard ($1, Android), that lets you do just that.
It replaces your Android device's keyboard with just one button that emits just one message (spoiler alert: it's "I am Groot"). There's also an Android app called Hodor Keyboard ($1) for, you know, the Hodor fans out there.
What's this app good for? It's totally up to you. Like that one app that only says "Yo," this simple, succinct message is completely contextual. When your friend's, like, "Hey, where you at?" sometimes the best (and only) answer is "I am Groot." It's an app that's almost as good as dancing baby Groot. Almost.