Hangrails

What is the purpose of the hangrails (or hang rails) in a file cabinet? And, when purchasing a new file cabinet (Vertical, 4 Drawer, Metal, Locking) for home use do you need to purchase them separately or are they normally included with the cabinet? Or, do you even need them at all? Thank you.

Where can I find hangrails for a 36" 5-drawer Anderson Hickey laterial filing cabinet? I am looking for the hangrails that go inside the drawers so you can hang hanging files. But thanks for answering. :-)