With iCloud and Dropbox (and SaaS in general), everything is linked to your user account. They assume that each computer is used by exactly one user.
But that just doesn't match my reality! At home we have a shared computer in the living room, and at work we have build servers shared by multiple people.
I really don't want to sign in with my personal Apple ID or Dropbox account on a shared computer.
With Syncthing that doesn't matter. I can just sync a single folder, and syncthing doesn't care who that folder belongs to and doesn't require granting access to unrelated folders that happen to belong to the same user.