The strain relief is for machine made cables where they glue or melt the relief into the plastic plug. Almost all manual crimp connectors have a strain relief

The strain relief is for machine made cables where they glue or melt the relief into the plastic plug. Almost all manual crimp connectors have a strain relief built into them that's meant to grab the jacket and hold the cable right at the entry to the plug. All my RJ45 and RJ45-EZ connectors in my bag have this locking strain relief tab at the back.