I don't know for sure but I believe it came about after a series of swaps in which one person did not fulfil their side of the deal.
I'm all for good manners. I suspect that as we all swim in a fairly small pond here and recognise familiar and trustworthy names, a feedback thread isn't really necessary to enhance a swapper's reputation. I think people are just wanting to show their gratitude and give kindness some kudos.
If a swap comes up with someone who is 'unknown' or keeps a very low profile, so you might not know if they're trustworthy or not, a thread like this in which they can be commended is a helpful reference to anyone with whom they may wish to swap goodies with for next time.
When it's just us regulars, a private message with "cheers, it got here safe" is enough for me. Getting glory in public for what was meant to be a discrete wee kindness makes me uncomfy. Grateful, all the same, but my ego is secure enough without it