Simple. Some times people either are uncomfortable downloading and burning CD/DVD discs, so they purchase ready-to-run media, or they don't have an adequate burner. IE, you can go online or to Best Buy and purchase many Linux distributions... :-) What you pay for is the manufacturing and distribution for the most part. It may cost you $25-$30 instead of $100+ for Windows, etc. Ubuntu used to have a free media program where they would send you free the entire system on CD/DVD, though you had to wait for awhile. I actually got one of those once just to see. Naturally, mostly I download and burn (or create a VM) the iso images. A 4GB live DVD takes me about 2 hours to download, and 10 minutes (and a $0.50 disc) to burn.