While the parallax knob is often used to 'focus' the target at different distances. That is not the actual purpose of the function, and is actually an incorrect use for it. The parallax adjustment is meant to put the optics in the same plane as the target. Which in turn makes your sight picture parallax free.
With iron sights, the cheek weld is even more important than with modern optics because there is no way to make it parallax free. If your cheek is in a different position than your last shot, your looking through the sights from a different place and your not aiming at the same spot any more, even if you line up the sights correctly. You can test this by using your fingers as mock sights. Line them up in front of you so that one tip is directly in front of the other. Notice the spot that you are 'aiming at' in the background. Now move your head without moving your body. After you line them up again, you are not aiming at the same thing again. This is parallax.
With the parallax option in modern scopes, that is no longer an issue as long as you adjust the setting to where moving your head around doesn't move your cross hair on the target. Once this is done, no matter where your eye is in relation to the scope, as long as you have a full sight picture, you are aiming at the same spot.
As long as your target is clear enough that you can aim at it, you can be just as accurate with it out of focus. Parallax on the other hand can be the difference between a hit and miss. Just because you focus the image using the knob, doesn't mean your parallax free.. and vice versa. Those two optical properties are mutually exclusive. Being parallax free is the more important one.