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It helps keep snow off the stuff in the back of your truck. That's the main reason for them.
If you're asking if the weight will help with traction, it depends on the topper. Aluminum ones are really light. They won't do much at all. Fiberglass can weigh 200 - 300 lbs, so that will help a little.
Like Bob said. I keeps the snow out the back of the ruck, which is a HUGE help to me since i use my trucks as trucks. It sucks have to constantly shovel the bed out or trying to get frozen things out of the bed