Because when you make stuff for yourself, you won’t have to wait to find an audience for yourself.

You can iterate and improve it fast because there will be a real user from day one.

It is also smart because when you make stuff for yourself, you also make it for those others like you. 

Then it’s only a matter of them finding you or you finding them.

Which is not too difficult and simple enough to learn.

So in a way by making stuff for yourself, you set yourself up for success.