Neck range of motion is important in swimming. The problems manifest themselves in:

1. Forward head on neck where the neck is stuck with the head in a forward of spine position. This puts your head in a deeper than necessary position which makes breathing hard.
2. If you cannot turn your head enough on either side, then you may not easily turn to air. It could mean that you will need to compensate by turning your body more. However, the more vertical your body angle is, the more likely you will sink which will make it even harder to breathe. As you turn to air to vertical, you actually start sinking which takes your mouth further from the surface.
3. If your neck is not mobile and movement of your head is restricted, you may find that the head moves back and forth with the body as it rotates from side to side. The head wants to maintain the forward position relative to the chest due to the stiffness of the neck. This will increase drag when swimming. We want the head to be looking down the whole time and motionless as the body rotates from side to side.

What is a good range? You should be able to put your nose in line with your shoulder on either side. If you can get a little more behind the shoulder, that’s even better.
However, note that your head should be on the neck and aligned with the spine when you test this. If you need to tilt your head in order to get your nose to line up with your shoulder, that doesn’t count! So posture is important here, and needs to be restored as you work on neck mobility.
Watch this video on some techniques to increase neck mobility: