On a colder day the air is more dense, so more energy (work) is required to drive the compressor. If no extra fuel is added the engine will slow down.
If you don't want your engine to slow down as temp drops, then a little more fuel is added to keep the engine at the same RPM.
(the upside of this is, you are now producing more thrust as more air is being passed through the engine even though RPM remains the same)
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