Who does and why?
Did you have a problem before you started doing it?
Did it produce the results you wanted?
Do you do it only on bullets with cannelures?
From what I understand the neck tension despite being very important is a consequence of brass quality and bullet size. I.E. the brass will resize to the size of the bullet when seated and the amount of tension is dependant on inherent strength and flexibility in the brass in that case neck.
I can see how softer brass could produce less neck tension and harder brass produce more.
I get how any variance neck thickness can produce different tension just through having more brass in the neck
I am not sure physically how a crimp can impart more tension unless the bullet has a cannelure in which case it is forming a tighter "lip" in the groove and ultimately
restricts your seating depth to that level.
Or the crimp is slightly crushing the bullet jacket in which case either the neck is going to be slightly thinner there or the bullet is going to have a microscopic waist from the crimp.
thoughts from the educated for one eager to know more....