It gives you perspective. A lot of people combine the two processes, and there's nothing wrong with that, but when trying to find the workflow that suits you best, it's always constructive to weigh the alternatives.
If you master as you go, the borders of mixing and mastering become pendulous in a way. If you separate the processes completely (perhaps mixing through a quick loudness chain to get an idea how the dynamics will sit in the final product), and hopefully take a bit of a break after the mixing stage is done, you can focus on the mastering stage with a different mindset. Personally, this is how my brain and ears work best. I get the mix where I want it, take some distance to the music, and come back to the mastering phase treating the project like it was just another mastering gig mixed by someone else. Of course, in this case I do have the luxury of fixing problems in the mix if needed, though.
Perspective is one of the reasons I always, always recommend outside mastering. Not just because I make money from it - I actually frequently recommend bands to use someone else for mastering (and I'll gladly recommend my colleagues) when I'm in charge of the mix. Not because I couldn't do it, but because the other person might hear things that I've grown used to when shaping the mix.