the common automobile is design to release its heat build-up in the engine bay via under the car,but if idling for too long, eg. stuck in massive jam,,heat naturally goes upwards,therefore temperature rises in the enginebay dramatically...one reason to jack up ur hood.
another is cuz the timing belt of an evo engine is too beefy to be housed in a proton chasis,hence ppl jack it up to clear it like what good ol buddy fugz mentioned...
BUT better be thinking twice of jacking up the hood because u will weaken the nuts n bolts as its lengthen to satisfy ur bengness taste. In a case of head-on collision, driver or passenger may be beheaded by the end of the bonnet