It’s instant applied RR generally on your attacking skill so is prized for the same reasons as people like taking Oathkeeper for Celestial Presence versus something like Thermite Mines. On a pet user, i imagine you’d also be able to apply it in a wide area with a point in Bone Harvest and Dread for example.
The only non-pet build I recall that uses Guardian Gates for the baked in RR/ADCtH was thejabrixone’s Fire Forcewave Tactician from pre-FG. Nowadays though, more options exist with FG so it’s niche is less exploited.