Its a courtesy to the sow really. It also cuts down the chances of the piglets injuring one another while fighting for nipples. Ive had to patch up piglets that got injured by siblings while fighting for nipples too.
Ive noticed this issue more so in LARGER litters than in smaller litters where the piglets need to fight for nipple placement. Not so much on litters of three or four piglets.
Some sows don't mind getting bitten at all, some sows will get bitten once and will never want to feed their piglets again after that. I have one sow where the piglets almost bit a nipple clean off and she was still oinking and singing while she fed them.
Some people never cut their piglets teeth and that's their choice. I had 17 litters before I began cutting teeth on piglets and on that 18th litter (larger litter)the sow got very injured and almost lost a nipple and I had to keep going in and patching up piglets from nipple fight injuries. Ive cut teeth on litters ever since.
Also keep in mind some breeds of pigs have different types of teeth than other breeds. Kune piglets aren't going to have the double canines that a juliana piglet will have.