1. I think most people submit to OCR because it's pretty much THE video game remix site...I don't mean to sound like I'm brown-nosing, but there is literally no other video game music site as well-executed as this one. It's a central location for downloading and discovering video game remixes. Your alternatives are to try searching for and sifting through thousands of Newgrounds/Bandcamp/SoundCloud/MySpace pages scattered throughout the nethers of the Internet, which is pretty much what you have to do with doujin sites. So basically, OCR is perfect for artists trying to expose themselves to a considerable, and more importantly, targeted audience.
2. Whenever I send a track to OCR, I am essentially guaranteed four reviews of my work from people I know and respect, plus one from DJP if it gets posted. There are other reasons regarding why I arrange and share music in general, but beyond the aforementioned virtues of the venue, the quality of feedback is the main reason I submit to OCR specifically.