> Is this a useful device to get a feel for creating music, or is it only really useful if you already understand music theory or improvisation?

"Useful" is a really subjective word when it comes to making music in general. Some template groove on a shit keyboard could be the base for a hit song (like Gorillaz' Clint Eastwood) while others don't make stuff unless in front of a modular synth with 100% control of everything.

Any instrument can be fun without knowing music theory, as long as you're willing to trial and error for a bit, until you find what sounds good :)