Why and how do some people use circular knitting needles when working flat / not in the round?
I personally only use circular knitting needles. To work flat on circular needles, you do not need to do anything special. Just turn them around when you reach the end of your row, as you would normally, never join. If you think of it as two separate needles with a string holding them together, it may be easier to imagine how it works.

I believe there are quite a few advantages to using just circular needles:


You only need a set of circular needles rather than a set of straight needles and a set of circular needles. 
You can't lose the "other needle". 
Once your work starts getting bigger and heavier, it will rest on your lap instead of on the end of your long needle with seemingly increased weight due to the lever effect.
You don't poke the person next to you.
You can get long cables that support big items like blankets.
Various brand of circular knitting needles have removable cables. This allows you to move the knitting to the cables and remove or change the needles. This is especially helpful in a few small ways, for example picking up dropped stitches with a smaller needle becomes easier, but you can also store your knitting away on the cable white reusing the needles.