The user can speak into the phone soundlessly, but is still understood by the conversation partner on the other end of the line. As a result, it is possible to communicate in silent environments, at the cinema or theater, without disturbing others. Another field of use is the transmission of confidential information.
For the transmission of passwords and PINs, for example, users can change seamlessly to soundless language and, hence, transmit confidential information in a tap-proof manner.
Link via Popular Science | Photo: US National Gallery of Art