Tax Loss Harvesting - question about the reality of it

I understand generally how TLH works. It seems like it's being over-marketed by some reputable institutions and used as a buzz word.

Can someone explain to me why it's a "good" thing? I mean, if you're portfolio is doing well, why would you sell good performers and take a loss on the bad. Yes, you can buy the high performers again after the 30 days, but it doesn't make common sense to me for some reason.

I hope this is the right place to ask this and TIA!