Well, the main reason is because it's free.
You don't have to join to use this site, but you must login or register to see exclusive move-in specials, real-time prices, detail floorplans, oversized pictures, virtual tours and more...
Creating a member lets you "stamp" properties. This marks the ones you like, so you can come back to them later. Also, it lets other people see which ones you found interesting, so it can help them, just like it helped you.