We always camp with canvas tents. The other kinds don't shed water as well, or at ALL. We have never had to put a plastic tarp on top of our tent to keep us dry.
It's not to keep dry, But rather to get the snow to slide off.
When I leave a tent set up for 6 weeks, there may not be anybody around when the snow gods dump 36" of new snow overnight.
For me it's a 2 hour drive in the truck then a 90 minute horse ride to get to my tent after leaving work on a friday afternoon. I really hate arriving at my camp site for a weekend of hunting to find my tent has collapsed under the weight of a Monday snow storm. Because if it does, I don't care how good the canvas is, A tent laying on top of cots will leak water and you will have wet sleeping bags. Not what I want to find when it's dark and 10°F outside.
If I was staying in camp, I could go out and shake the snow off after a heavy snow and the canvas would be just fine.
I don't use the wall tent in the summer months. Too much and too heavy to pack. I do just fine with a nylon dome tent for summer camp outs. But in October during hunting season. We can have 70° days or 0° nights. It can be warm and sunny or blowing blizzards of snow. We pack in a cylinder stove to keep the tent warm. Cots to get us up and off the frozen ground to sleep on. I want a place to come into and warm up after spending 12 hours tramping around the mountain looking for elk or deer.
I can open the window and let a little air in if we get to hot a fire going in the stove. Or I can let the smoke out if the fire gets smoky, wet or green wood or something.With a ridge pole, I can hang wet cloths up to dry over night fromt he heat of the stove. And of course I can cook a hearty meal on the stove. If a storm blows in and we have to hunker down and stay put for a day or two. At least I can standup and move around while we play cards or pass the time. In a small dome tent, You get feeling really cramped fast if you have to stay put.