--It is an easy way to share music with a couple friends who aren't tech savvy. Installing it was easy for them and I was able to say "just drag it from iTunes to your dropbox folder" and it worked. Other ways of sharing were too complex.
--It keeps software that doesn't have built in syncing synced. I use it to sync 1Password and more importantly (to me) PersonalBrain between my home and work computers. It works fast and flawlessly.
--It's an easy way to transfer files from home/work. Sometimes I'll download an app or something that I know I'll end up redownloading again at home. With dropbox I just throw it in there and when I get home I don't have to find the download link and wait for it to download, I just open the folder and there it is.
--It keeps a few files I use all the time available from anywhere. Mostly ebooks, a couple videos and a few documents and templates (resume etc.).
Overall though it's the ease of use and reliability that keeps me using it. Even if it could do all that but was a pain to use/install I'd probably still be searching for other solutions.