It is one way to help improve a guns accuracy, but not the only way. They are generally shunned, most hunters and shooters won't touch one. When they show up on the used market they are hard to sell and usually go for quite a bit less than a comparable gun without one if they do sell.
That doesn't mean they don't work. They do help somewhat, but most don't think it is worth the ugly protrusion stuck on the end of the barrel and the increased muzzle blast. If you handload you can do the same thing by tweaking your loads instead of the barrrel.