Not just redirects. (There isn't any built in with XMG)
When you migrated from vBulletin to XenForo, even if you retained content IDs, there's still the chance that users could have been merged or other behaviour.
This gives the case where user_id 123 in vBulletin could be user_id 456 in XenForo.
The import log would record:
type = user
old_id = 123
new_id = 456
If the old user ID 123 had hundreds of albums and images in PhotoPost Pro, they will be recorded against the user with ID 123. If we didn't refer to the old vBulletin to XF import log, they would be imported against the old user ID. This would leave that user confused as to why their hundreds of albums or images were missing. They'd be missing because they'd be logged against his old user ID.
There is of course still room for this to happen. If, since moving to XenForo, user_id has been merged again with another user and his new user_id is 789 then we could see the same problem. But that is something that is probably even rarer than the first case.
But aside from that, it's still important we get the import log in case the content IDs were not retained. In that case you could have users getting no albums, someone else's albums... Which would be a disaster.
If there was ever a special case where there was no import log but someone still wanted to do the import, I would provide limited help with this (as long as retain IDs was used and the potential issues were understood) to help the import still go ahead. But otherwise it would be impossible without the import log.