The main reason police use them is to make it harder for a BG to grab the officer's gun and use it against him, and that could also apply to anyone carrying a handgun openly.
As a practical matter, if there is no, or too little, retention, it is possible for the gun to jump out of the holster when the carrier has to run, jump, crawl, take cover, or otherwise engage in activity that could cause the gun to come loose from the holster. I know that some people will say that the gun should always be in the hand at any sign of trouble of any kind, but in most situations outside "cowboy country" that is not practical and could be trouble.
One of the reasons my favorite holster for many years was the Bucheimer Federal Man was its great thumb break system, but the trigger guard was exposed rather than being sealed under seventeen layers of immovable plastic, so it would never pass "lawyer" muster today. Modern holsters seem to be designed to NOT allow the gun to be removed other than by the ME prior to the autopsy.