There are many reasons Animals reproduce. There are also many ways Animals reproduce.

Your question is going to open you up for being taken for a small child in need a Sex Ed. The school systems appear to pick the same kids over and over again to take classes while others go without,.. then that is how alot of Teenage Pregnancy comes to be.

One type of Reproduction is refered to as an animal being A Sexual. In these cases an Animal gives birth to usually an Identical Clone, but mutations still happen ( this means they are slightly different, not that they have Teleknesis or anything like that). These Animals tend to either lay eggs or they are too small for the human eye and literally rip apart into two identicle bodies. They CAN also give Birth but this is rare.

There is something called Live Birth that some Fish are capable of. This means their eggs are fertalized in their body but not dependent or attached to their body and after they hatch the mother OR FATHER will release them when they are safe. Anotherwards they are already alive and out of their egg before they are released to the world. Seahorses are an example of sea life that is both Live Birth and the Father gives Birth.