When a thief takes a steak from the cooler and walks out the door, they don't know who that person is. And while they may have video of parts of this, they don't necessarily see enough to prosecute. (Acting like you're stealing a steak but not actually doing it isn't a crime. Shoplifting can be hard to prove; part of that proof means demonstrating that they didn't change their mind and just drop off the steak somewhere else in the store.)

When a thief takes a steak from the cooler and walks it up to self-checkout and pays for it as if it is a bunch of bananas with their credit card, they have identified that person. They have them on video at the self-checkout committing this crime.

It actually doesn't matter much if they leave the store with their bounty or not in this second case. The crime is already done by converting the steak into bananas.