Well you would normally use hammers and nails for woodworking but you wouldn't be able to use woodworking to make hammers and nails... But I guess with a text editor it's a weird recursive thing where the programming tool is itself a programming project. I think people just do it for fun and because they can. You'll probably never make something you can productionize but maybe you will learn some cool things along the way.

The same reason as all the other projects mentioned, because it teaches you many useful concepts that show up elsewhere when writing code. A carpenter likely won't learn much by manufacturing their own hammer that they can apply in their own workflow, a blacksmith on the other hand might.

It's a nice self-contained project that's relatively easy to make fast progress on, while also being challenging. Plus it's something that programmers have to use every day so we're bound to have pain points with existing options.