The waiting list serves several purposes. Firstly, it’s your chance to show your interest in a future holiday prior to the holiday being advertised.
Occasionally, a holiday is advertised as ‘booking soon’. Adding yourself to that holiday’s waiting list gives you the first opportunity to reserve a place when booking opens.
Finally, if the booking has been closed due to the holiday being sold out, then adding yourself to that event’s specific waiting list gives you the first opportunity to step into a booking if someone changes their mind.