The optimisation opportunities available to the compiler author changes depending on where you are along the process. As you go from the original source code to

The optimisation opportunities available to the compiler author changes depending on where you are along the process. As you go from the original source code to the final binary, you progressively lose the ability to reason about high-level constructs and gain the ability to reason about low-level concerns. What these extra IRs afford you is more granularity. More points to choose from along that high-level/low-level gradient mean you also have a better chance at the right balance of high-level and low-level information for your optimisation pass to work.