The one evening a short trip can give
The booked evening is the single night of a short break that carries the session, chosen early so that the choice still has names attached to it.
A weekend appears to hold two free evenings and in practice holds one. Whatever else the trip turns out to be for, one of the nights goes to that; the other is the booked evening. Naming which is which, days before the flight, is the single decision this whole site is arranged around.
Why naming it early is the point
A decision left until eight on the Saturday competes with every other visitor who also left it. The same decision taken on the Wednesday is answered with a choice of names rather than with one, and it can still be moved once for nothing if the weekend rearranges itself. Early is not about urgency; it is about the answer having options in it.
How it interacts with the front half
An evening named before you travel is automatically more than twenty-four hours away, so half the fee travels first. That is the mechanism rather than a cost: it is what makes the evening yours. The booking terms set out what that half does if the plan shifts.
Leaving room around it
An evening with a fixed obligation an hour after it is not really the booked evening. Ninety minutes plus an arrival window plus the quiet afterwards asks for roughly three hours of nothing else, and a trip that cannot find three unclaimed hours in two nights is a trip that should book on arrival day rather than on the fullest one.