[coder543]

That might work for physical product, but when you claim your software download is sold out, people tend to give you funny looks.

You can definitely put up a product page where users can enter an email address and start building up a mailing list, but obviously mailing list recipients are not guaranteed to pay you money.

[drenvuk]

The purpose is to get some kind of signal that you're on the right track. Many times I've seen would be founders just go off of a hunch without a buy in from any possible users of any degree. Even their voluntary email sign up is far more of a sign than what the founders know users want.