Keeping my house in order helps me to keep order in the rest of my life. Plus, I'm more motivated to keep up with the cleaning when it's an ongoing process. Once it piles up and becomes a mountain of nastiness, I don't want to mess with it.
I'm also much more content and happier in my home when it's clean. It feels more like "home," not just the place I sleep and keep all my stuff.
I also don't have to scramble to straighten up if someone comes over, because the apartment is already nice and neat!