I cut this out of another post because it is completely accurate but I wanted to expand on it.
"In any case, when you shave, three days later, you get three different lengths of hairs. One long group that are the growing hairs in Anagen Phase, a second shorter set of hairs in shedding catagen, and a group that may only be stubs against the skin, in telogen resting phase." Quoted from James Walker
Another reason I ask clients to shave or trim their hair is if the hairs overlap and I have to spend time digging for buried pores. This time could be better spent treating hairs.