For convenience and throughput.
Convenience is important. You likely have been frustrated trying to find the right MIPS instructions to use in your programs. Sometimes it feels like fitting square pegs into round holes. How much more convenient to have a wide selection of pegs, even if they don't add computing power!
Convenience is not just for human programmers. Compilers and other systems software output machine code. Compilers are easier to write, and are less buggy when there is a rich set of instructions.
Throughput is how much computing a processor (or full computer system) can perform in a unit of time. Say that a processor can perform 50 million instructions in one second. The more computing each instruction does, the greater the throughput. So most processors have instructions that do much more than the bare minimum.
Which of the following improvements increases the throughput of a computer system?