Best Answer
In simple terms, it is just showing off and is the riders way of saying "Look at me, aren't I great?" and it achieves absolutely nothing.
Back in the days of the old 2 strokes it used to be a way of keeping its throat clear as they were prone to choking up if left on idle for too long, but 99.9% of bikes these days are 4 strokes and will happily tick over on minimal revs.
You might also notice that in the main, it tends to be riders of the smaller or medium capacity engined bikes that make all the noise often fitted with a race pipe (often illegal) in the belief that they can get into a traffic light grand prix allowing them to show of the size of their pathetic manhood, whereas those of us who tend to ride the bigger stuff are quite content to just sit on idle as we know we have nothing to prove.
It is annoying I know, and riders do tend to grow out of it as they grow up, but it achieves nothing, serves no purpose other than to annoy those around them, and make themselves look stupid.
I speak as a professional rider of 45+ years and instructor for 40 + years 😊