zabbix items normally are polled at some interval. while this is ok for many metrics, for some it might make more sense to poll them less frequently.
for example, a system might have a static number of cores, max memory size and a few other parameters (some systems have these dynamic, our hypothetical system not ).
it doesn't make much sense polling these hourly. polling them weekly is a bit better, but if it changes after a reboot, it will take up to a week for zabbix to notice the change.
it would be nice to have some way to tell "poll this item only upon some condition".
for active agent items, it could be agent startup - but that wouldn't work that well with other items (like checking max memory size over snmp).
maybe a way to tie item polling to a trigger firing ? that way a "system freshly booted" trigger could make a bunch of items collect data, and we would not keep on collecting static data again and again.