There are a few great reasons for doing this. It makes your team less dependent on you, and it makes them feel empowered in that way. They learn your job is nothing magical and that they, too, can solve problems that they might otherwise just say "management needs to solve this". It erodes the "us vs them" dichotomy of management vs workers which has never been helpful in my view.
Another reason is that junior developers might not really know or fully understand what their manager does, and thus might not be considering it a valid career path for them.
Finally, it frees up some of your time, which you will find when you become a manager, suddenly becomes a lot more precious. With that freed up time, maybe you can spend some time coding, so you don't become too detached from what your team is doing.