"When it comes to storing and syncing files online, Dropbox's accessibility from multiple platforms, easy to use interface, free starting service, and reasonably priced professional services make it hard to beat."
Dropbox is one of several high-profile cloud storage and sync solutions, but it comes out on top due to its fantastic iPhone (and iPad) support, multi-platform availability, easy to use interfaces, excellent availability and reliability, and highly competitive pricing.
Apple's iCloud is a great way to keep your personal information, your device data, and a few, compatible files like iWork backed up and in sync. However, iCloud doesn't provide anything beyond that. If you want to store files strictly online, if you want to access and edit them from other platforms, if you want past versions of files kept available, if you want an actual filesystem to work with, then iCloud isn't enough. (Anything a previous user of iDisk on MobileMe or .Mac knows and laments.)
Dropbox does all this, easily and transparently, and more.