What's the significance of the Hash Key as it relates to multiple site collections?

The Hash Key value is used to encrypt data on the query string when an invitation notification is sent. This ensures the values of the registration URL have not been tampered with. If a value is changed, say the invitation id, the registration will fail. There is no impact of using the same hash key for all site collections. However, if you do change the Hash Key value while there are still pending registration, the pending registrations will become invalid. You will need to resend the invitations.