Pshop Assignment: "Save one selection as channel" - what does this mean? Help!

Question

I am in a Pshop class and have made a composite photo using 7 photos and layer masks.  Here it is so far.
In the assignment it says "Save at least one selection as a channel".
I know I can select a fish, for example, and then Select / Save Selection and it will save it as a channel.  What I don't know is why I would want to do this.  Could someone explain what Saving as a Channel is and how I would use it in this pic?
Thanks,
Gary

Answer 1

JJMack,
Thanks for the reply.  I think I am understanding but still unclear about a few things.
I make a selection in the main document and Select / Save Selection.  When I click on the Channels tab, here is what I see.
I know I can now fine tune the selection via brushes or whatever.  What I don't understand is what I can do with it at this point.  If I do a Cut / Paste to put that fish in another document, for example, all I get is the mask.  If I want to duplicate that same fish in this document, again all I get is the mask with a cutout where the fish should be.  So on one level, I guess I know "how" to make a selection for this project.  I just don't know what to do with it now that I have done it.
I have read other tuts where they say this is a great way to save your work - like if you are cutting stuff out but not done and will come back to it later.  But I know I am missing something because once selected, I don't know what else I can do with it.  And now that I am doing things in layer masks, it seems like cutting stuff out is not the preferred way since it is destructive and that using a layer mask is better because you can always go back to the original and just modify the mask if you need to.
Thanks for anyone's responses.  I was fully understanding Pshop up until this point.  Saving as a Channel baffles me.
Gary