Bill Houghton, Sebastopol, CA
The cap iron (odd term used deliberately) serves at least three functions on a Stanley plane: chipbreaking*, mating with the depth of cut/coarseness of shaving adjuster so that it can work on the iron, and providing somewhere for the cam on the lever cap to work against since said cam would otherwise fall into the slot in the plane iron**.

Some literature I've seen also argues that the two irons together are stiffer than one alone; I've never seen any reference to a definitive study of that.

I don't see how you COULD use a Stanley bench plane without the cap iron.

*I am stipulating here that it does fulfill that, while being ready to find out that it doesn't.

**Which slot exists, of course, both to accommodate the screw that holds the cap iron on and as somewhere for the lateral adjuster to fit, so it can do its work.