That was the original style of engine nacelle on the 757 before the decided to duct the whole thing in a common nacelle.
If you look closely this engine nacelle is the same style on the older 747-200s that BA
flew as well as the RR
powered 747SPs and of course, the L1011.
I think it was just to streamline the engine...because as you can see all RR
powered 757s 767s and 747s now have the ducted RR