They never know when they might need them. If disk space gets cheaper and cheaper, it changes the trade-offs you make. Your time is worth more than buying another disk. People now expect near-instant access to data all over the world because of the Internet. Caught in the middle are the bandwidths of the computers and the networks.
People get angry when they click and it's slow.
When you click on a Web page, it might go halfway around the world and talk to hordes of databases and machines. You don't care. If it's slow, you're mad. The stress falls on the data management in between and the ability to transfer data at a reasonable speed and the ability of software to manage huge numbers of files. In the next couple of years, the explosion of disk capacity at low price is going to cause a lot of trouble for systems administrators who have to worry about disk backup and about interpreting huge amounts of data.