In some situations, it may be useful to temporarily acknowledge triggers.
Imagine the following (classic?) setup: actions defined with escalations to send an alert every 15 minutes, as long as the problem persists and is not acknowledged.
One would usually notice the problem, ack the trigger and fix it. But there are times (in case of many triggers firing for example) that some are less urgent, and the notifications (by SMS) are annoying.
It would be useful to "acknowledge" the trigger for a fixed period (say: 30 minutes, 2 hours, 4 hours, ...) after which it's automatically un-acknowledged and the escalations start again.
The alternative is that the person who did ack the trigger remembers that he still has to fix it (since it's ack'd and no escalations are being sent out), but that doesn't always happen.
If this appears as a useless feature request, feel free to trash it of course - for us, with many hosts & triggers, this is an added value to be able to ack a trigger for a fixed amount of time.