What is the reason for the complication of organisms in the course of evolution?

In the course of evolution, random mutations have occurred in various organisms. Beneficial mutations and adaptations were passed on to descendants. Weaker organisms died. Thanks to this, the complication of the organism took place. But simplifications also took place in organisms. For example – a man has lost his tail and third eyelid. We no longer needed these parts and they gradually disappeared and turned into rudiments. The goal of evolution is to obtain qualities and properties that will help the body survive without the presence of complex physiological systems.