Because you don't have to use the same identity you use to post notes articles like and send money as you do for the identity you use to zap mulvad to fill time.
Discussion
I have dozens of npubs, but even then.. what’s the benefit of using any npub as the identifier for a VPN service instead of a random 16 digit number? I strongly advise everyone to always create a new account with Mullvad and not to do a top up.