Without device sync, if your browser updates, or you exit the browser, or you restart your computer, the browser will often clear its storage.

We don't want to store the ability to spend your money on our servers in plaintext, so we store them in your browser. Your browser will purge its storage unpredictably is the downside.

With device sync enabled, we encrypt your spending credentials so we can store them on the server rather than in the browser. This tends to make the credentials more available (absent bugs like the one siggy saw this morning).