There speaks the man who never had to share files with twenty different people, each using different computing platforms and of varying technical competence.

Cross-platform internet file sharing in a transparent way is a (surprisingly?) hard problem. Before Dropbox there were many companies who had tried to make a success of it and they had all failed[1] in one way or another. (Not cross-platform enough, not seamless enough, reliance on ads for income etc etc.) DropBox succeeded because they took that hard problem and made it look easy.