Some places wont insure a roof unless the old asphalt shingles are removed.
^^^^^
I know of contractors that will not warranty the work or the material's if they are installed over any other roofing material.
Because they can rape the home owner for the additional cost of stripping the old shingles.
Note, do not lay steel directly upon the aggregate surface of asphalt shingles, as the aggregate will scrape through the paint and zinc which will allow the steel to rust.
