A pre-drawn handgun reduces the "get stabbed before I can react" range dramatically. The general rule of thumb is you need a knife threat to start at least 21 feet away to draw, aim and fire(See - Tueller Drill). When already at the low-ready that range drops down to ~10-14 feet, if I recall.

So if you might need to defend yourself, but you want to avoid escalating the situation by aiming a gun at someone, it makes sense.

Because pointing a gun at someone is a very dangerous thing to do. It massively escalates the situation. At that point, if they or anyone else in the situation thinks they can shoot first, it becomes a real option and is also the only way to definitely protect yourself from the gun-pointer. Even if it's not a real option, people under that kind of sudden pressure don't think straight. A police-officer who habitually points a gun at people in situations that don't justify it is a danger to themself and everyone around them; the unlucky person staring down the barrel, other police-officers and unlucky bystanders in the wrong place at the wrong time.

Once a gun is pointed at a person, there are no more steps of escalation, no more ways to express how much you want them to do something; the only next step is to pull the trigger. The situation has suddenly become extremely dangerous and very volatile.