They should start firing as soon as you've set a local description, whether it be an offer or an answer.

Think of it as an optimization: partial updates to the localDescription. If you wait a couple of seconds to inspect localDescription then the SDP will contain all ICE candidates already, and you won't need to listen to any events. It works to just send the SDP and ignore these events. But this is slow.

To speed up connection establishment, the initial localDescription provided is incomplete, missing ICE candidates, since they take time to generate. This lets you signal the SDP early — unblocking the other end — provided you promise to follow up and send the missing candidates as they are generated (which is when the event fires).

Yes, onicecandidate event fire as soon as we set offer or answer to localDescription but it will not fire the event till we wouldn't create the datachannel or any streaming channel first.