Phone turned off
If you are in a remote location and you forgot your phone at home and it is turned off, you could still use the text function from an internet cafe or from a computer.
You can always use GoogleVoice or web hangout in that case (texting directly from web)
As some are saying, the pointof Mightytext is to use the phone as third party to send text from web to phone to recipient...
This is what Gtext is for... texting from your Gmail account using your Android number.
Not possible with text messages as the message has to be delivered to the phone so the phone app can push it to the website. Same with your phone being in a no service area.