I'm not a Trezor user as I personally prefer Ledger Nano S so I really am not sure about this, but this is probably because naming addresses, and accounts requires a bit of storage memory. My guess is that the Trezor doesn't have that much memory for these data to be stored so they require you're dropbox account to be connected so the data can be stored there.
Just my guess. Someone correct me if I'm wrong though.
Thanks for the explanation! That makes totally sense. I also guess that this should be safe. The guys from Trezor seem pretty competent in what they are doing and i don't think that they would be stupid enough to store ANY sensible data on Dropbox!
Hi All
For people who might be reading tis thread as reference, it is also for portability, since a person will be using his Trezor wallet securely on any computer, using Dropbox will insure you will be able to see labels and notes whenever you move from one pc to another..