Because ___domain names are cheap, and feeling like you are actually producing something that the world can see is a great source of motivation. You also run into problems when you deploy to a real ___domain that you never encounter if you're just noodling around with code on your local computer.

The risk of creating a bunch of toy programs on your local disk that you never show to anyone is that you avoid the hard parts. Accountability is a good thing, in programming and in homework.