Hi tasmaniantiger, and welcome to the forum.
Dropbox can be a handy way to have your projects saved to a cloud-storage account. It’s useful if you’re working on your project from multiple computers. Or, it can be a good place to save any manual backups you create using the File > Back Up > Back Up To... command in Scrivener.
By default, though, Scrivener will already be creating backups automatically each time you close your project at the end of a writing session. Did you open the project, write, and close it three times today? Then you’ll have three backups from today.
For my main writing projects, I work only on my Mac. I have test projects I sync between my Mac and PC for troubleshooting as a L&L staff member. I have the iOS app on my iPad and iPhone, but I rarely write there either.
For years, I wrote all my first drafts by hand and transcribed them into Scrivener for editing.
As rms said, syncing is good for those who need it. But, not everyone does.
For securing your work in multiple places, this forum thread titled Got a Backup? might be helpful. Some of its tips are Mac-specific, but it’s a good starting point for considering what services you have and need.