The reason it gets to see the unchanged will as well is because if it's in a setup with a third party roles such as werewolf or killer then they might put useful information in their wills because they take the chance they will not be killed by mafia.
This role works as a hooker for wills so making the real wills appear when it dies would be like the hooked cops reports appearing when the hooker dies if the cop is still alive and this role is not very powerful because town can win without wills.
When this role dies the town can trust their wills again just like when the disguiser dies the town does not need to guise test anymore.
This role makes Janitor useful in will setups and a town Drunk could try and stop the Forger from changing the will and you still have roles that can investigate or reveal such as oracle that can find the mafia.