After freelancing in agencies, startups and design teams for so long I saw that everyone was using Dropbox for projects and design file storage.
With designers making hundreds of changes to their designs each day, the time it would take to keep your prototype updated is prohibative. So we figured instead of forcing the user to bring the files to our cloud solution, why don't we just go to theirs?
If we let users leave their files where they are it's one less click and thing to manage. We just keep the files in sync so the prototypes update automatically with the user lifting a finger.
On the flip-side, no everyone uses Dropbox of course and it's a barrier. But it allows us to save money on storage which we pass back onto the user and provide a great free plan (no upload or project limits)