Why are analogous organs considered  to be evidence of evolution ?

Analogous organs give evidence evolution coming from two directions.Some biological characteristics are analogous, which means that they serve the same function in different species but they evolved independently rather than from the same embryological material or from the same structures in a common ancestor. An example of an analogous structure would be the wings on butterflies, bats, and birds.

Another important example would be the development of a camera-type eye in both mollusks and vertebrates. The fact that eyes in different species are analogous structures proves not only that the eye could evolve naturally, but that it, in fact, evolved several times, independently, and in slightly different ways. The same is true of other analogous structures as well, and this is because certain functions (like being able to see) are just so useful that it's inevitable they will evolve eventually.