When do we use all pins in RS-232 cable?
The RS-232 standard was originally specified to support connections between Data Terminal Equipment (DTE) such as computers, teletypes and video display terminals and Data Communications Equipment (DCE) such as modems and Automatic Calling Units (ACUs).

At the time, DCE did not have any internal intelligence, so dedicated signals were designed into the RS-232 specification to manage specific features that were common to such equipment, such as flow control, on-hook/off-hook status and call progress.

Nowadays, modems have their own microprocessors, so it's actually easier (and cheaper) to ignore the dedicated signals in the RS-232 connector and do everything over the serial data lines, using the ubiquitous "AT" protocol.