If you dry fit the headset in place and find it to be 1-2 mm short, a headset mill can accomplish two things. One, it will allow the cups to seat into the frame correctly, aligning them and ultimately all the pieces in the headset. The second is that the milling process will remove paint and a bit of material.

Done correctly, the mill will remove the same amount of material from both the top and bottom of the headtube.

Also, take care when installing the cups. While a section of 2x4 and a hammer can be used to put the cups into the frame, a proprer headset press will install and align the cups correctly.

This could be the difference between a good fit and a precise one.