Question

why do animals have certain useless organs in body

Answer 1

Animals generally don't have organs that are useless.
Every organ is useful either for maintaining temperature....and much more.

Answer 2

All organs are useful in animals body.But there are some certain organs which are once useful and became useless during the course of evolution.For example, consider males,in them mammary glands are useless and in human beings hair on skin,diaphragm etc are useless.But in herbivores,diaphragm is very useful.