It's actually good when sold several month or a year later.
It will cut some large roots, at the cuts, little feeder roots will grow out, making the tree easier to adapt and recover after transplant, also, getting the root ball easier into the pot with more feeder roots.

Yup, the big roots do nothing for the plant, its only the tiny hair like ones that do the job of taking up nutrients from the soil. When you cut back the big ones, the plant immediately starts growing the small ones again to make up for the loss, meaning the plant actually acclimates to it's new home faster than if you hadn't.

Kind of like trimming a hedge. The more you cut back, the more you encourage it to grow (up to a point, obviously).

I'm sorry to be the odd man out here, but I think for the most part a nursery offering to root prune is simply a sales gimmick.

Some people that buy trees are so unfamiliar with trees, they're just plain scared to prune anything. To them a pre-pruned, root pruned tree sounds like an valuable service pushing their decision to order the tree from that nursery.

I've never read any research that suggests a root pruned tree will grow faster than one not.

I agree with Olpea. I don't even think damaged roots are an issue or that clean cuts are important. Nature is constantly playing little tricks that open wounds on roots but from what I've seen, the odds of meaningful consequences must be awfully small.

They might say they are root pruned,...just before they're sold and you're getting some cut sticks.

I happened to me one's on a cherry tree, no feeder root's, just a stick in a pot, it never grew.

Big roots anchor the tree. But if you are going to dig the tree, you will lose the anchor roots any ways.
So root pruning gives you feed root to recover the plant.
A healthy tree can then send out more anchor roots to hold the tree against high winds, down pours & small boys who climb them for fruit.:-)