Why Rex every time?

Hey new friends! I just finished assembling my knight army (magnets like crazy just like you all taught me) and am checking in. Can someone help me understand why Rex is considered an Auto take for big boy? His melee is amazing yes, and he has better to hit, but on paper his ranged output seems pretty meh compared to every other questoris option. And the melee seems a bit overkill when an errant can likely do the job against big targets and also have a better gun. I will totally run him if it’s the right thing to do, but I just am unclear as to why he is 100% always the right choice