This option must be activated in order to create an After Effects Live Link to Cinema 4D. After Cinema 4D is restarted, it will scan for connection attempts from After Effects. The connection can only be established by After Effects (but can be terminated from either application). See also
Details about the After Effects Live Link can be found here.
Cinema 4D uses this port to communicate with After Effects. If conflicts with other applications occur, a different port can be defined.