I guess I am just trying to think through the logistics of this. You go online, tell them you want to go through security on this day and time. On the day, you leave your hotel, you know you need to be at security at X time, so you allow enough time to account for small issues, meaning you likely arrive early to the security area...so instead of getting in line and waiting your turn, you stand around in a "non-line" until your appointed time, then head into the line for reserved timeslots, and you go through the normal screening process.
I feel this must be a reaction, or fix, in response to a couple years ago when security lines at Schiphol were hours long for a period. At Schiphol, and most all airports I have been to, including Heathrow, while yes, a line, it doesn't ruin my day, I get through in plenty of time, from what I recall less than what time I might allow to "be there on time for my appointment". So maybe, if I hear or by using the planner, see that the wait is a couple hours, this might be handy, but most of the time? Another example of a solution in search of a problem.