Originally it was for sound deadening and heat transfer, also to kill and rattles or other funny noises that the roof can make. You did the right thing in scraping it all off, might want to consider putting some seam sealer or something like it wherever the braces can come in contact with the roof. Last roof I did on the inside I coated the inside of the skin and crossmembers with some of the Lizard Skin heat deflector. It was a dark colored car and anything that will help kill some of the heat is a good idea!!!!