Lynn, I definitely had the same question when the issue first arose. I think the idea is that the people who don't have a photo ID, typically a drivers license, are disproportionately old, poor, or minorities. If they don't have the ID, they will be prevented from voting. I don't really think that its impossible for those people to get them if they have the time and money to do it. Its an extra hoop for a largely disenfranchised group to have to jump through. From a practical and statistical standpoint, they are much less likely to already have the ID. The rest of us who has IDs have no extra hoops since already have an ID.
The backdrop of this is important, though. Voter fraud is believed to be very low. R politicians have been clear that the rational for the requirement is to reduce voter turnout in a way that is good for them.