From a US regulatory standpoint having an operational product before a token sale is really important. The Howey test is the main standard the SEC uses to decide whether or not a token is a security or instead a utility token. If there's no existing product use for the token then the SEC has strongly signaled that they will consider a token a security because the only thing people are buying is the expectation of future profit.

There's some good literature out there on the Howey test that's worth a read.