What is the Point of PCIE X1?

Hello.

Whilst reading this evening (a book about computers) I was reminded that each of my motherboards is provided with a couple of PCI Express x 1 sockets (together with one each PCIE x 16 socket).

What I should like to know is what are the x 1 sockets for? What plugs into them? Why are they there etc., etc!

Thanks
Paul