@Kuram: If you want to see an example of what Terisa is talking about, have a look at
this screenshot. That shows several books with series info. And the recent firmware versions support sorting by the series info in the lists. Very useful if you have a collection for each series.
So the device does have the concept of series, but it doesn't have the concept of "series collection". Using the "series" column, you are creating collection that just happens to have the name of a series. It doesn't have any different behaviour to my "Testing" collection which is a list of books I have put on my devices to test some something, usually a bug.