I've had helicopter flight instructors come to me with the written already done. The FIA can be taken without an endorsement. The AFA requires an endorsement. So it was helpful for my previous trainees that wanted the written out of the way before beginning training with me.
Also, if you choose Sheppard Air for test prep, they offer prep for the FIA and not the AFA. So they wouldn't provide the needed endorsement for AFA. Could be worth the extra 75 questions if a trainee wants to knock it out prior to training with their CFI.