It's to protect consumers who see what looks like a good price and don't notice the discrepancy. (A friend of mine whose budget is very tight has been caught out by this a few times, and is the one who pointed the law out to me.)

For a while, my closest grocery store (part of a local chain) was terrible at keeping pricing information up to date. Occasionally I'd get surprise discounts, but more frequently I'd get charged too much instead. I was going to buy those things anyway, but on the principle of not letting the store screw people over, I started being a stickler about it. And guess what? The store shaped up.

(I assume the mechanism was that the increase in the number of price overrides at the register caused a review, and the store manager allocated more employee-hours to keeping prices in order, instead of treating it as an optional task.)