The only time you required a blanket was during a wet start. After an initial failure, normally by using an incorrect procedure, the aircraft would be towed to a dry part on the dispersal. The blanket was to protect the tailplane from the billow of flame out of the jet pipe and also to stop any fuel still adhering to the tailplane from lighting up.
I had to do it once, at night. It was quite spectacular. Never got the start procedure wrong again.