It is for the orange VAT with plexiglass. Pulling of the FEP film from the plexiglass directly might cause some problems (I have read it in the forum before).
But there were actually various reasons for me to apply the FEP over the PDMS: a) solution for orange VAT (see top), b) easy to get off and to realign if necessary. This cannot be done with a glass surface as well. I have had problems with bubbles in the past on glass, but it just might be my clumsiness, c) no modification of the MSM as distance to projector is the same as with PDMS only and d) no modification of the "Home" parameter (build table is at almost same height). Given all that one can also switch easily between FEP and PDMS only.
My 2 cents,