The primary objective of processor designers is to improve performance. Performance is defined as the amount of work that the processor can do in a given period of time.
Different instructions perform different amounts of work.
To increase performance, you can either have the processor execute instructions in less time, or make each instruction it executes do more work. Increasing performance by executing instructions in less time means increasing the clock speed of the processor. Making it do more work with each instruction means increasing the power and complexity of each instruction. A real-life analogy would be to imagine you are pedaling a bicycle. To get where you are going more quickly, you can either use a low gear and pedal very quickly, or use a high gear and push harder.