For use case I encountered personally, imagine i.e. dependent discovery, relying on another item that is throttled. In normal run, it's all ok, and when there are no changes, no discovery is necessary.

But there are times when we are creating/modifying/debugging stuff, and have i.e. removed discovered items/triggers manually, and need to rediscover them anew right now. 'Check now' will not help us there because item is throttled, so we have to either wait for automatic discovery when throttling interval expires, or remove throttling, wait for configuration interval, do 'Check now', and then place throttling back in place again.

The same applies when we modify some dependent items preprocessing, or modify triggers, and want to recalculate these dependent items / triggers immediately. 'Check now' for the parent item will hit its own throttling and so our changed dependent items will not get updated and changed triggers will not get fired.