For me I often have some technology I want to learn that's tangentially related to work (I'm a software developer), however I can't justify the time to learn whilst I'm at work, and don't want to work on something work related when I get home. Sorry for overuse of the word "work" there but, fundamentally, outside of my job I find fiddling with data a lot less compelling than making objects fly around the screen and blow up.
So generally it's as a way to motivate myself to learn something that otherwise I might not bother to do. Examples:
- JavaScript
- node.js
- MongoDB
- Azure
- three.js
- WebGL
And the list goes on.
I really like old arcade games so I tend to find it quite fun to build my own versions of them, which I usually add my own twist to. But still, I quite like the aesthetic of older games like Defender, Stargate, Space Invaders, Galaga, and of course the vector games like Star Castle, Asteroids, Battlezone (and Star Wars - who could forget that one?), so I tend to go in that direction with the games I create. Still, I do tend to modernise in the sense of wanting very fluid controls and a more forgiving difficulty curve.
Anyway, the point is that doing this has motivated me to learn some technologies where otherwise I might not have bothered, because I'm often happy to work on this stuff when I'm tired/fed up/am unmotivated to do any "real" work.