It also makes it easy to find all your subscriptions in one place. When every provider tries to get you to subscribe, hunting them all down becomes a pain.
Eh, your account is still tied to an instance. That instance stores your subscriptions, as well as the vast majority of your preferences. If you want to get way into the weeds, that's why Jerboa won't persist comment sort order, but other clients will - there is a server side prefer for all/local/subscribed and new/hot/best at a post level, but not at a comment level.