I am glad someone finally asked this question. I have used gaiters while cross country skiing but never saw a need while hunting but have been curious why I see and hear of so many other people using them while hunting.
They'll keep the bottom of your pants dry in wet conditions. When you walking through wet foliage it makes a big difference. Also when crossing streams they'll help so the water doesn't go up and get the inside if your boots wet and your socks.
i use gaiters when shed hunting, I'm usually wearing shorts and running shoes but still in rough country and the gaiters really help. they're awesome in snow for keeping it out of the top of shoes/boots. protect the bottoms of you pants too, thats usually where my pants start to wear first.
I've been using the new kuiu gaiters with good results. even my granola friend wants to hang up his trusty OR gaiters for them.
I wear OR Crocodile gaiters and have had great success with them in all terrain and conditions I've encountered; high wet grass, knee deep river crossings, snow, scree, postholing, sidehilling. I wear them all the time.
Like many have said, they keep the crap out and the warmth in around the lower leg.
Sidehilling on the decent out of the Talkeetnas last year, OR Croc gaiters.