It's really just a matter of balancing the difficulty of creating a new "identity", so that legitimate users can occasionally use multiple identities to partition their traffic and make it harder to get doxxed, but are still deterred from creating identities cheaply to engage in sybil attacks or escape blocks. There are various ways of committing real-world resources to an identity to deter such abuse. Actual meatspace identity is of course one way of doing this, but there are probably others.