Hi,
Interesting question!
Should have been a discussion 
<but why I find some channels are set in Automatically mode without any Time Planning for them?>
For Sender adapters, this is as good as manual mode when channel is started and will follow the Poll interval set in ID.
For receiver adapters, this does not mean anything since message being sent is driven by source. I don't think the message will be persisted if we set ATP on a receiver adapter for a particular time.
Hope I was able to answer your question.
Ambrish