In the pursuit of your hobbies, you will find problems. Many of those problems can be solved (or made less onerous) through coding up a toy app.

Or you can tackle the really hard problems if you want. Make a CNC lathe for wood working. Maybe create an app to layout your cuts in the most efficient way.

I don't know enough about crotchet to suggest anything...but I bet there are problems to be solved.

As far as why you wouldn't use an already existing app? Why would you build a birdhouse from scratch when you just go buy one?