It can do harm, because some chancer can step into the gap to queue-jump, and the person responsible for the gap in the first place might not even notice if they are engrossed in their phone or whatever. I have myself taken my place in such a gap inadvertently once or twice (in front of a person who I took to be browsing the adjacent shelves), but I was corrected and I apologised; it is best if the issue is avoided entirely. There is also the matter of the tail of the queue creating a greater obstuction than it needs to, especially at supermarket checkouts where the tail typically extends into the main internal cross gangway.People in queues -- for... whatever is being queued for... who when there is a gap between them and the person in front of them in the queue: omit to step forward and close the gap up. Being behind .... makes me feel frustrated and angry. Will admit that this is highly irrational on my part: the thing does me absolutely no harm ....