Does it really need to be done in advance?

It's an Advance Passenger Information form.

So yes: it does need to be filled in advance. How far in advance, and who it needs to be given to <b> depends on where you're travelling from and where you're travelling from </b>

As a general rule, governments require the information before the departing plane takes off. With most airlines to most destinations requiring an API, APIs for te whole plane can be given to the government of the country you're flying to as long as everyone completes them by the tome they've checked in.

But waiting till then holds up checkin queues - and increases the likelihood you'll get a trivial detail wrong, which will mess you up when you present yourself to Immigration. So most airlines strongly pressure you to complete te form online, either as soon as you know where you're staying afyer booling, or at the very least when you checkin online.

And the airlines are quite right about this. It's the height of bad manners to make fellow passengers wait while you're completing a form you could have done months earlier.