Holding consistent tension on a subsection while cutting helps to:

Maintaining consistent, even tension on each subsection of hair while cutting helps produce an even, accurate cutting line. Uneven tension — pulling some hairs tighter than others — causes an uneven result once the hair is released and falls into place. Tension should also be appropriate (not so tight it distorts the natural fall, especially around the hairline). Controlling tension is a fundamental haircutting skill, and understanding its effect on accuracy is part of the cutting theory tested on the exam.