It's not super useful for big stuff, but nice if you're trying to write stuff just for yourself.
Reasons are:
1. You just install the client and it works.
2. The files are backed up, while your ftp solution might not be.
3. Your space can grow depending on disk prices without you doing anything for it.
4. A lot of possible merge conflicts between different computers are automatically merged by their smart tooling.
[0] https://instant.io
* Very good online file sharing UI
* And no need to run and secure an FTP server
If you use a local copy, you have to set up something to sync it with the cloud version, or sync it manually.
Dropbox keeps a local copy for speed, and also syncs automatically. It's the best of both worlds.
It's also easy to use.
It also keeps a file version history.
It's especially useful if you use multiple devices