The computer is pretty much essential to get the catalyst working fast, and to keep it working. Most of the "allowable" emissions on the test cycle are emitted before the catalyst gets to working temperature: there was talk (may now be becoming fact) of having to pre-heat the catalyst to working temperature before allowing the engine to start.
Opinions vary widely about the validity of catalysts on cars. I spent too much of my life in Brussels on this very subject (years ago). Catalysts were "mandated" by bureaucrats, possibly under pressure from one lobby or another. The general consensus back then was that there were more efficient ways to achieve the same result. Nobody could tell now - the requirements have moved on, and nobody is doing research on non-catalyst technology.
But the computers to drive all that stuff have their fringe benefits on digital toys for the driver
