The advantages of re-roofing vs roof over:

There are advantages of stripping a roof down to the sheathing when preparing for a new roof. The first is to allow the roofing contractor to re-nail the sheathing to current code. This reduces the likelihood that the roof and building structure would be damage during a storm and may provide discounts on insurance. Once this is done, the roofing contractor will apply an underlayment prior to installing the roof product. In some jurisdictions, a secondary water resistance (SWR) barrier (peel and stick) can be applied directly to the sheathing for an additional insurance credit. In theory, if the roof product were damaged or missing during a wind storm, the SWR would protect the home preventing water intrusion into living space.