Denny Board

1988 build. Denny Board used as sheathing under concrete tile. Framing over spanned. I have not seen this in 25 years. Some has been replaced with cardboard and some missing. I have seen tile installed with felt only but most new work is on sheathing. Educate me. Pics

The structure doesn’t look beefed-up enough for concrete tiles. Wonder if this is not the original roof house plans call for ?