Let's say we have 100k people that live in a few poor counties where not everyone has government ID and they can't afford to spend time / money to go very far. Now, if 40k of those people need an ID to vote, reducing the hours that the nearest DMV operates will reduce the number of people that get an ID.
Now, you might say, "but why can't they just go to the DMV in the next county?" or "why can't they get their IDs well ahead of time?", and you'd have a valid question! But when you have 100k people.. creating what looks like a minor inconvenience will definitely translate to fewer voters in the end. Now, lets create a bunch of minor inconveniences and the numbers really start to shrink.