it's a slow, i/o operation. Perhaps it takes a second or two; maybe you're generating a PDF and the email is of non-negligible size, or your email provider is slow that day, or something of that nature. When a user clicks a button, and the response is just "ok, we sent you an email", you can just issue that response immediately, and then send the email. Otherwise, the user's browser just sits there and hangs until the entire process is done.
