Gaiters keep gunk from getting into your shoes (dirt, rocks, sticks, snow, what have you). They are especially good in snow, which can have a chilling effect as it goes down into your footwear.
They keep mosquitoes from biting through your socks. My first trip in trail runners (after switching from boots), I forgot this little issue (the gaiters were in my pack!) and got many bites around my ankles! Driving 900 miles home with constantly itching ankles was, to say the least, interesting!
They keep your pants dry when going through tall grass wet with dew or rain.
If you're wearing shorts, gaiters protect your lower legs from stinging nettles, poison oak, thorns and other interesting flora that can be found at the edge of (or even in) the trail.
In spring gaiters keep ticks from crawling up your legs inside your pants. You can achieve the same effect by tucking your pants into your socks, but it's a lot harder to keep them there.
Gaiters aren't essential, just handy. Get a cheap pair of low gaiters and try them out. If you decide you don't like them after trying them, no big deal.