There is two way to look at this,
1.I cover my sailboat to protected it from the snow, the weight of the snow, the ice and the water that freeze and unfreeze twice a day in spring. It also protect my hull from the sun while I don't use my boat.
2. I don't like to cover my boat because the ropes that attach the tarp on, and the tarp can flutter/Vibrate in hight winds and scratch my nicely polished hull. All the snow will eventually melt and just exit by the drain. And I don't like to have no/reduced ventilation in my boat because it encourage all kind of stuff I really don't want in my boat.
I covered my boat a few years, It really is a pain to always tighten those ropes and make sure no snow builds up on the tarp.... Two years ago I didn't and it was so windy that there was no snow on the boat, not even in the cockpit. So we did the same thing this year, but this year there is way more snow so it did build up in the cockpit, now thats a pain in the *ss to go up there (she is on the hard and a hour and a half from my home) and shovel the snow of her. No snow on deck tho, so i think next year I will cover the cockpit only, I'll build a wooden frame and put a tarp on top.
Both ways have advantage and disadvantage.
I hate storms, but calms undermine my spirits." - Bernard