Unice, I believe that this is because other hair removal treatments will interfere with and give an incorrect assessment of where your face actually is in terms of facial hair growth. By avoiding other facial hair removal for three months before hand, the facial hair you arrive with there will be a "normal" full wave of facial hair. You can continue shaving during that time and stop shaving 4-5 days before your first appointment.
Below are the before and after for session 3, that I had on April 4, 2014. Of note to me, I am seeing significant change in the upper and lower lip area between session 1 before and session 3 before. The chin and jawline are still pretty dense.

As a reminder to myself, costs were over $1100 for the first session, over $1000 for the second session, and $751 (and change) for the third session. The first two sessions involved two techs for 5 hours each (10 total hours) and 4.5 hours each (9 total hours) respectively. The third session was actually harder because it was one tech but I was in the chair for 7 hours total. There were breaks in there, including an hour for lunch, but total actual chair time was about just under 7 hours. So there was maybe 25% less hair total to be removed than during the first two sessions.
My next appointment is May 30th. I'm eager to see what the before photo looks like then.
I will note that my face has been smoother this time for longer than each of the first two sessions. I also took a few hours the second evening back and used tweezers to pull dead hairs that had broken off. It's very easy to tell which are dead and which are not. The live hairs still hurt like the dickens when yanked! The dead ones don't hurt at all, and occasionally slide out so easy that it's funny. I got about a hundred dead hairs then couldn't find any more, though I did yank 6 or so live ones along the way.
I will also note that now, a week after this session, there are still zero beard hairs showing this time yet. There are lots of very fine and small facial hairs, that many women often have, but no beard follicles yet. The process does appear to be slowly whittling down the number of facial hairs and HRT may be affecting new growth as well.