As a test for product market fit? This product is more meant for notifying users when your product is ready to be used and thus is launched :)

They would say just launch something, and don't make a big fuss and run-up to some date

And perhaps waste weeks/months of development time because you don't even know if there is interest in the product? using betanotify you can check by the amount of visits/sign ups what the amount of interest is

it's fake interest though, just build and release early. Not saying spend months, but beta signups to a non-existent product is false signaling. People do this because they have nothing to give up. It's like VCs not saying no because, who knows, maybe they will pull it off soon and I'll take a look again then.

Get whatever it is in the hands of people, better to talk to strangers in person than to do a beta email list before starting building.

It's a waiting list creator for a SaaS product you might be creating