what is this expansion bay used for on the back of a PS2?

This can be used for a hard disc and a modem/network card.

i wont be repetitive here, the hard drive is being sold but only online on select sony affiliate site for like 100 to 200 dollars. the network adapter is out and is 40 bucks, there might be extra charges from your isp though.

oh, so thats wot it is then. and for a minute there, i thought it was something interesting