Years ago, their desktop client was the only one among popular similar services that supported proper delta sync while also covering all major platforms. Absolutely indispensable if you used it with things like TrueCrypt containers, where changing one byte within would cause most sync clients to resend the whole 50 GB, or however large it was. Dropbox handled this fine and would finish in a couple of seconds (actually sending new data, not just pretending it did that).

I don't know how it is these days, wouldn't be surprised if other commercial services still haven't figured it out.

Google Drive added delta sync this year, while OneDrive added it in 2020 apparently.

I also overstayed with Dropbox for that reason, and now I don't see a real reason for their higher $/GB. Though their client is more stable than Google Drive from my experience (which randomly stops working on Windows often enough), OneDrive has been rock solid for me on both Windows and macOS.

I do this with a WebDAV server (Fastmail Files) by breaking up my encrypted volume into 10MiB chunks, so that the sync has the granularity of that, plus a FUSE script to present the directory containing the chunks as a single block device for mounting. Obviously not cross-platform though.

AFAIK that's still their USP.