Wet grass getting your trousers/socks wet.
Stones, grit, pine needles getting into your boot and socks.
Plus, combined with a decent length jacket/cagoule it often means you needn’t bother with waterproof trousers, so get less hot and sweaty.
As for “what gaiters?” almost anything does the trick with the breathable fabrics making a slight difference. If you have skinny ankles I can recommend the Mountain Equipment gaiters. A close fitting gaiter is best in snowy/icy conditions when you don’t want to catch one with your crampon and go head-over-heels 🙂
FWIW, I was wearing some short “scree” gaiters over my MT90 boots whilst cycling at the weekend.