Query: 2 cables in 1 junction box
What you have there is a perfectly standard, and to modern code, switch loop.  And every wire is where it belongs. 


  It seems that the fixture is using the red (as the live?) from 1 cable...
  If the above is correct, why was this necessary?  What is the red wire's purpose - where does it originate from? Were the 2 blacks also live?


You're puzzled at why if the red is live, why are the blacks there too?  Let me ask you this: how does the light know when to be on, and when to be off? Do you want this light to be on at all times?  

That's right.  You need a "hot" wire that is on, only when you want the light on.   That is called the switched-hot and that just gave it away. Ideally, it's nice if switched-hot is red. 

Now where do you suppose the switch gets hotness from?  It's not magic.  It gets it from another wire, that is always-hot.  The switch either connects always-hot to switched-hot, or it does not. Always-hot is usually black. 

What about neutral?  Neutral returns the current to source.  (sorta like hydraulics, you plumb a return pipe, or you'll get a huge oily mess all over your shop floor.)  The lamp needs neutral, it goes between switched-hot and neutral just as you see.  Code requires neutral be white (or gray, but you'll never see that). 

Now you are thinking.  Wait.  Why does a switch need neutral?  That switch doesn't.  But if you ever change it to a smart switch, it will, and accommodating those is a 2011 Code requirement. Switch loops must be /3 cable: 


neutral must be white
always-hot is conventionally black
switched-hot is red because it's possible here. 


And as it works out, this box is perfectly color coded, the colors always match the functions.  That is often not possible (though it's always fair game to color-code wires with tape). 

The lamp doesn't need always-hot, so it just passes on through.   The left /2 cable brings power in, the right /3 cable to the switch is a spur.