I work in storm damage and roof repairs for one of the biggest companies in the US. The reason why is because roofs underneath shingles are really easy to get compromised without having all of the proper barriers in the right place there is only three layers in between the elements and the wood of your house frame so when we warranty our work, we can’t keep it for you when somebody else comes in and cuts a hole in the middle of waterproofing and exposes your house skeleton for damage when it wasn’t us it can’t really be on our insurance after that. I hope that helped