When working from the holster, I don't need the BUIS. When working from the safe, the briefcase, the tabletop, or even when doing a reload that's in a non-normal position, the irons somehow let my brain figure out the position of the gun, in space, and help to get it squared up, so i can find the dot easier.
Obviously, dot failure is a whole nother thing. I can use the "horns" on the RMR and line them up with a torso shaped target and do pretty well.