Thanks for the input so far, I never thought about it just being used as a barrier between the shingles and the tin for abrasive block. I was only concerned about the water leaking from either rain or condensation. But that's why i wanted to get other input. I am not a proclaimed builder, carpenter or roofer. but I did build my own house, took 3 1/2 years but i did it slowly from ground up and passed all inspections through the county, when building inspector came out periodically.
ALSO:
During my conversation with this roofer as to why he did it this way, the subject of stripping the tin also came up. you know putting wooden strips between the tin and the sub-roof, and he stated he was against that also. His reasoning was that tin will condensate when cold air gets under it. and those bigger air gaps created by the stripping would let the tin condensate easier/quicker. ??? again i do not know if he right or wrong just seen a use of felt that i thought was improper. and this curiosity and conversation led to other things, and before feelings got hurt i quit and went back to feed'n cattle with a few more questions than answers and thats why i put it out here.