When a company chooses to operate in stealth mode it provides optionality in a couple of ways. First, you can better control timing of press announcements since the world doesn’t know what you are doing yet and you haven’t shared anything before (more appealing to a reporter). Second, it gives the company an option to figure out more about the market, iterate on the product, or pivot with lower risk.
Of course, none of this works if it’s already well known what you are doing and you don’t deviate from it.
If you are in stealth mode, it’s probably best to maintain strict confidentiality from the beginning, especially with investors as information tends to leak. Even so, it’s unlikely that you can be successful without talking about what you are doing to a large group of customers, investors, or subject matter experts.