The rollback feature allows sysops to quickly undo malicious or malformed edits, such as spam or accidental blanking. Since it undoes everything done by the most recent user to the point where someone else edited, it should be used only with caution. All automatic rollbacks are marked as minor edits, and do not add the page to the user's watchlist.
Normal editing (review the page history, select a useful version, and edit from there) can be used by any non-sysop users to accomplish the same goal, albeit with more clicks and taking a bit more time. The undo feature, visible when comparing two diffs, can also sometimes be used.