There's really no reasoning behind it, it's just easy to use, and integrates well.
How do you get Free Space? by sharing http://db.tt/ISG2U8Eb
There's also something to be said about having *up to 16 gigs of storage for free.
For every person you invite, who joins and installs Dropbox on their computer, they'll give both people 500 MB of bonus space (up to a limit of 16 GB)!
Once you've invited friends, you can view the status of your referrals
Linking with Facebook and Twitter, Follow @Dropbox on Twitter as well as Posting <3 to Twitter gets you free space. And don't forget to complete the Getting Started for the extra free space!
If you need even more space you can always upgrade from there.
By: Jeffrey T. Kastner