Rumble 150 issues

I have Googled and found a post on this forum re Rumble line out signal.
That was interesting and helpful. That is the problem with the one I have.
But I tried to cross check with headphones attached and I was not getting
a head phone signal. The headphone jack should cut off the signal to the
speaker, right? If now what is the point? and what is the point of line out
that also won't disconnect the amps speaker. The point is I want to hear
it through the mixing board to computer software to mixing board to monitor,
So if there is a signal from the speaker as well it conflicts with the monitor.

Admittedly I am a novice with this. But I can use some deductive and inductive
logic.
Thanks for time and attention

Update: I have the headphones working:
They were cutting off the speaker. I have a cheap faulty set of
headphones I was using. And I am guessing that I need to pipe
the headphone signal to the mixing board if I don't want the speaker
on the amp. I was thinking that the line out jack with a cable connected
would disable the amp speaker. But maybe that is not the intention.

 

Thank you for the reply:

What are the more common uses?
I want to record the signal from line out and monitor the recording before and while
recording.