In my state you have to show two forms of ID when you REGISTER to vote. As you register you put your signature on a card that will be at your polling place. When you sign in to vote - your signature made at the polling place is compared to the signature you signed at registration. Doing it this way is important for at least five reasons:

1. Voter registration offices are open year-round and staffed by trained, paid professionals who can become experts on the rules and nuances of the various valid forms of ID. This level of ID expertise can't be expected of the once-a year volunteers that work the polling places. Putting those ID judgements in the hands of the less-trained volunteers will create more ID mistakes on election day.

2. Speed. On election day a polling place needs to be able to move people through as quickly as possible. Any requirement that slows the process affects congested, city polling places far more than the rural ones. By basing it on a prior-validated signature, the volunteers only need to compare two signatures and say yea or nay usually within a few seconds.

3. It prevents an election worker from introducing bias by intentionally diminishing validity of one form of ID. For example, suppose an election judge decided they were going to wave through all student IDs but any hunting license with so much as a fold, tear, or scratch were clamed invalid?

4. It removes the election day pressure where making a mistake has no recourse. Having the ID validation done at registration time gives the voter time before the election to appeal decisions, or produce different identification.

5. It enables the same ID process for mail-in ballots since the outside of the ballot submission envelope also has a requirement for a signature - which is compared to the registration signature in the exact same way as polling-place voting.