A passkey uniquely identifies you as an individual donor and is associated with results that you have completed. Previously we did not have unique identifiers in Folding@home (usernames for example can be shared by many people). One main benefit of passkeys is that if someone cheats, we will not need to zero out the points of everyone with that username — only the offending party and those without passkeys.
Perhaps more interestingly, we have introduced a new Quick Return Bonus (QRB) points program, which is tied to passkeys. To earn this bonus, you must use a passkey.
Finally, the existence of passkeys opens up future possibility of displaying your individual credit and other information, rather than just by username, so use the same passkey for all clients.