| Either:
1 - to increase the engine speed to such an extent as to ease the meshing of gears when changing down so blip before change(i.e. the lower gear is teeth are moving faster than the previously selected gear teeth - layshaft and main shaft). Much like trying to grab onto the waltzers when they're already whizzing around, it's a damn site easier if you run around at a similar speed.
or
2 - The blip is required to notify the forward traffic of your intention to perform an overtaking manouevre, much like the flashing of headlight to notify other road users of your presence. Particularly effective when using termis | 
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