To solve problems.
Now in my early middle age, there is no way I would learn a programming language unless it could solve a problem I have now, better than what I’m currently using.
Gone are the days when I would learn a new language just for the kicks.
However, if you do have a lot of hobby and learning time on your hands, it makes a lot of sense to dabble with other languages.
The more techniques and knowledge you add to your brain’s “inventory” - the better you can solve problems when they happen.