it's an optional step that you can take that helps with the clarity of the beer, AND allows you to age your beer for long amounts of time without fear of the yeast causing off-flavors.
Repeat, this is an optional step. Palmer shows you the easiest way to brew a batch, and that just involves a primary fermenter and into the bottles (via the bottling bucket.)
WHat a lot of us do is let the beer ferment in the primary (glass or plastic), then when fermentation is basically done, we siphon the beer out of the fermenter and into a clean new one (normally glass), and we leave all the sediment behind.
The beer can then sit for long periods of time in the secondry fementer (from weeks to months, even years) before you siphon it into the bottling bucket and bottle it.
as a beginner, I suggest doing the easy way for the first few batches. this is just one more step in a process that you are just learning about. Get a few batches under your belt, and when you're feeling confidant, use a secondary fermenter and see how it affects your beer.
happy brewin!
-walker