The standby-only parameter is a requirement for the backup interface when configuring interface redundancy. The XOS Command Reference documents this requirement for the backup interface in the following note (Chapter 7, Commands for Configuring Interface Redundancy):
NOTE: The specified interface must be configured to pass traffic
between the X-Series Platform and an external network. In
addition, the specified interface must be configured with the
standby-only (conf-intf-ether context) command.
Using the parameter standby-only ensures that the interface doesn't process traffic unless the master interface fails.