The reason they have reservations even though they're almost never fully booked is a few years ago (10+ at this point?), there was a situation on a national holiday on the mainland where many people booked tours to visit HKDL, IIRC including bus transport across the border and tickets to DL. Too many people showed up and they closed the park to additional entries that day, including people that booked tours and came to HK specifically for that purpose and had valid (undated) park tickets. Not only was it bad press, it showed an impossible situation for tour operators and other people: there was no way to guarantee that you could get into the park on any particular day, even if you had a valid ticket. The only way to prevent that is to go to date-specific tickets, or undated tickets that require a reservation to use.