Just having an idea is often not enough. I've got plenty of them, but still, there is something missing: a connection between the right ideas.
A single idea is not always worth doing. Or not complete enough on its own. It needs something else to go along with to make it worthwhile. Something that completes it or makes it deliverable in the correct way.
I might have an idea of doing something technical, learning a new programming language or framework for example. But following those age-old hello world examples all the way to the de facto todo app doesn't really excite me.
On the other hand, I might have an idea of a game. It won't be playable just like that. Writing down the idea just doesn't make the cut. No matter how detailed it is. You need the rules, the world, the story and the medium.
But those two don't necessarily always match. Not every game is suitable for any platform. It needs to be the correct one to make most out of it. And to think of different variations of platforms I could use it's not just easy to pick the correct one.
Sure I could just implement whatever I have built on a new platform to test it out. Especially if it hasn't worked on the previous one. But what would be the fun in that? I want to go ahead and start building that next idea already.