When ATC ask for and are given an "LDT" what do they understand that they are being given. What do they expect of the PIC when he reaches that time?
As an ATC I am expecting that you will leave hold at that time and either proceed to destination or divert to alternate.
Reason/logic: At your LDT, I believe you have enough fuel left to continue to original destination, make missed approach and then divert to alternate with appropriate reserves for that alternate (but that's pilot stuff). After your LDT you do not have enough fuel for all of that, thus you must divert unless you are assured of landing at original destination; it's your call.
If the reason for the hold is traffic, I wouldn't expect you to get 'promoted' in the queue. But if it were due to WX, fog etc. a quick negotiation (within company) can occur if we have the LDTs of all; this can also happen with traffic, but it's harder.
Why do we really want it; because we can make plans for 'generally speaking' a new flight plan or start wording up other sectors that you'll be on your way to XXXX soon. Tell the Flow to perhaps not count you as a number or be ready to remove you from the sequence.
Also we may stack you further out, or closer to your alternate if the time to LDT is large, with a view to bring you nearer when we are able; of course this may alter the LDT?
If you give me a LDT that is wrong, what can I do to assess it? If you say it's time 2315; I'm locking it in, if you revise it, then I lock that in instead.
Number one in all this, if you say we need to turn inbound now for a landing or divert now to XXXX I'll give you the option that I have available to me at the time.
I have had a few occasions where two or three minutes after a diversion the fog lifts and everyone can get in with minimal delay, because of earlier diversions, and the aircraft can't return. Also had the reverse. It's the old suck it and see principle.
Often when I ask for a LDT it takes a long time to get an answer, I understand it needs to be calculated/found; but a ball park would be handy as an initial planning tool; i.e. we’ve got 30 holding on, call you back with the LDT.
Often the handiest advice is the 'nominated alternate' and sticking to it. Very annoying if you reach an LDT and divert to a new alternate; not planned for etc. keep us in the loop.