The previous answer says I have to do something to get rid of weeds coming through. Why is that?

Remember that above all else you are putting this material down to save you from having to weed for the next 5 to 20 years. These materials are designed to do just that! However, there is sometimes an initial problem of aggressive pre-existing weeds, which must be handled first. When you have quickly and easily eliminated this short-lived initial problem, you will be set to enjoy the most effective weed control device available.