The Common Gateway Interface (CGI) requires that the program invoked via CGI (i.e., your script) must return an appropriate HTTP response to the web server, which is then passed back to the client (i.e., the user's web browser). Failing to do so will produce the all-too-familiar "500 Internal Server Error" response.

Or, coming at it from the other side, when the user clicks a button to submit a form, their browser takes them away from that page, so your CGI script needs to provide a new page for the browser to display. Even if the purpose of the script is just to access a database or run a system command, it's still appropriate to return a page stating that the operation was completed.

(Granted, the button could be an <INPUT type=button> or such rather than a traditional submit button, in which case it won't take the user away from the page and, unless you're using AJAX, the response won't be displayed to the user. In that case, returning a minimal response with just the Content-Type header is likely appropriate. But you still need to return something to avoid a 500 error, even if the error wouldn't be displayed anywhere other than in the web server's error log.)