shaving 2 to 3 days prior to treatment will allow the practitioner to be able to tell the difference between growing and shedding hairs. It would also make sure that all hairs are short enough that no time is wasted moving hairs out of the way in order to gain access to the hair one has selected to treat.
For those who choose to treat only growing hairs, the shedders can be simply plucked to save time, while still gaining the correct appearance. In that case, the practitioner would, say, treat one, and pluck three shedders near by. One would feel like one were being plucked on every epilation, but that would be because one is not feeling the treated hairs being removed, but one would feel the shedders being removed.
It is a treatment choice some utilize. It is just good when they share this information with the client/patient, as one who thinks they are getting plucked on every treatment might get the wrong idea and take the wrong action because of it.