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The gravel keeps the Sun, and its UV radiation off of the bitumen-based material that forms the (barely) waterproofing layer of your roof. Bitumen has poor UV resistance, and even the 'best' roofing felts come with only minimal guarantees.
Most gravel isn't the matt black colour of bitumen so reflects some sunlight, and absorbs/radiates the warmth from the Sun, rather than the bitumen heating up, expanding/flowing leading to thinning & holes.
The gravel also helps to displace (some) standing water from any low points.
Sounds like there is a territory-marking stand off on your roof.
mespilus
So I've just noticed that my felt roofed extension has a 50 50 mix of gravel and cat poo on it. Literally hundreds of cat shits of all sorts of ages nicely mixed and buried with the gravel.
Does the gravel serve any function? Can I just sweep it off and hope the cats find somewhere else to shit? Or what else can I do? I have two cats myself, but I think it's the neighbours cat as mine like to come home and shit in a box rather than outside.