The phaser probably has different settings to choose from.
They do, but in TNG and most of TOS they fire beams. In TOS they tend to fire stun or vaporize exclusively, with no mid-power kill. Season 1 of DIS plays to that nicely where everyone gets killed is vaporized. For some reason they stop in season 2. There is at least one instance of pulses fired in TOS, but they're more like brief beams fired in an alternating spray. Actual wide beam is a little iffy, but at one instance of stun looks like an area effect.
Yet the beams in Trek have never ever acted in a realistic manner. Why not just sweep a beam across and wipe everyone out? Instead they just fire shots which are too stable for any hand-held beam weapon. Try holding a laser pointer still and see what I mean.
Realistic is not as important as consistent for a fantastic setting, because phasers could act like ballistic weapons (supersonic projectiles following parabolas) one episode, and lasers (invisible instantaneous beams) the next, and in both cases they would be realistic, but horribly inconsistent.
Beam sweeping happens twice, accidentally. The angle of the beam is determined by where the user is looking, the beam locks at that angle relative the weapon while firing so it doesn't sweep around if the eyes move, but the hand can move the beam by pointing the phaser around. Twice, people's arms are knocked while firing phasers, and the beams sweep. One times it's Romulan programmed killer Geordi, another in, I think, DS9.
The reason they don't do sweeping in combat is likely safety training. As in, better to fire and cleanly miss than fire and accidentally hit anything unintended. It's more idealized cop than soldier, and I think it fits the idea of pacifist soldiers nicely.
