They were an FAA requirement in the early days of jet engines and pressurisation. It bypassed the need of the engine air being directly used for pressurisation and air conditioning, if you have any internal bearing chamber or compressor distress, it is possible to introduce smoke and/or fumes into the cabin, most undesirable. A T/C would, as correctly stated, draw ram air from the intake, compress it and sent it straight to the packs for A/C. If I too remember my 707 days at Gatwick, the covers were a sod to fit. Best method was to sit astride the monster and beat the latches liberally with the heels of a size 10 ! Oh, happy days . . . .