1. You probably have spare change or missing dollar bills in there. Throw it all in a jar and buy yourself something nice for your room when you are done!
2. Insects. The cleaner you keep your room the less likely that there are creepy crawlies hiding in corners, in your shoes, or otherwise populating a space that should be yours alone. Someone I knew hadn't cleaned for a long time, and found a black widow spider in her kid's toybox. YIKES! Insects are great...when they are outside, where they belong.
3. Keeping things clean is the lazy way out. Think about it: It is much easier to do a little cleaning every day than to spend a huge block of time sorting clothes and putting everything back. Usually when you clean a messy room all at once you are taking up a big block of time that you could be using to have fun. For me, it is just worth the few minutes daily to know that I won't have to waste fun time on keeping things clean.
4. When you can find all of your clothes, and they are clean and neatly put away, you can actually wear all of them and vary your wardrobe, which is very cool. This can prevent you from getting bored with your stuff, and even better, prevent you from wasting money on stuff you already have. Also, when you want to get a look at your wardrobe to see what you need, it is easier if you can get a quick visual instead of going by memory.
5. If there is order in your room, there is order in your mind. A messy room distracts you from focusing on what you care about most. Even people who like a lot of stuff, say a lot of artwork or collectables are happier when they are in a neat room where their collectables are displayed in an orderly fashion.
6. Less friction from those you live with. If you live with your parents, this is one way to keep peace in the house. If you live with a significant other, roommate or spouse it also helps keep the peace. The last thing you need is a reason for your home life to be less than peaceful. The amount of time you need to maintain a clean room is a lot shorter than having it out with the people you live with.
7. Cleaner rooms are healthier rooms. A lot of people are allergic to dust mites or dust itself. It's bad enough that your messy decor might be making you sick, but if your room is literally making you sick that can lead to feeling awful and possibly preoccupying your immune system so when a cold or flu bug comes your way, you might be too run down to fight it off as well as you otherwise would.
8. It is easier to get rid of old stuff you don't want if you can see it. Out with the old means in with the new, and sometimes giving everything a clean out and once-over means that you discover you have things you don't really want to hang on to. That makes space for things that are SO much more you. If you have stuff that you don't want that someone else might want, ebay or craigslist might be a way for you to sell your old stuff and make a little money.
9. When your room is neat, and you can focus on the details of cleaning, you can get a chance to wash your windows. They get dirtier than you think, and it might just add a dimension of brightness to your room you didn't expect, and for free!
10. People will be more inclined to hang out in your room if it is a pleasant place to be.
11. You deserve the best space you can possibly have. Keeping the place clean is your gift to yourself.
I will add this: If you are pressed for time or find the whole job is overwhelming, then give yourself permission to do a little at a time. Set your timer for 15 minutes and start with either one area of your room (desk, bed) or one category (clothes, papers.) If you can do one or two 15 minute sessions per day you will be shocked to see how fast the place cleans up. It makes sense because anyone will lose steam after 15 minutes of cleaning, so get in there, put your full energy into it for fifteen minutes, then pat yourself on the back and pour yourself a lemonaide!
Most of this data is from my own experience being a natural pack rat, but some of my philosophy and tips come from a variety of house cleaning books as well as flylady.net. Her email list can be too preachy for me, but her website is a great resource!