Reincarnation is a cycle of death and rebirth. After death the soul leaves the body and reappears again with another body. So only the body dies. The soul keeps on living. This process is needed so that the soul can learn the necessary spiritual lessons. When all the necessary lessons are learnt, there will be no more reincarnation and the soul returns to God.
So the goal is to end the cycle of reincarnation by learning all the necessary lessons. Ignorance leads to a perpetual chain of reincarnation. After many reincarnations, a person becomes dissatisfied and will seek spiritual happiness instead of worldly desires, so that he can free himself from the cycle of birth and death.
If these spiritual lessons can be learnt in one lifetime, obviously there would be no need for reincarnation. So they must be really extensive and difficult lessons. Some people claim to have memories of their past lives. We do not know the motives of such people. Most people do not have such memories. The total population of the world is increasing, which means all the humans on the planet are not the ‘reborn variety’. Many of the humans we see must have been born for the first time.
Those who believe in reincarnation also claim that some humans are reborn as animals and vice versa. They claim it is the bad humans who are reborn as animals and if they behave well as animals they may get another chance to be reborn as a human. So a bad human reborn as a cockroach need not lose hope. As a cockroach, if you behave well and learn the necessary spiritual lessons you can again be reborn as a human. Of course, we can’t hope to be reborn as a dinosaur, since they are extinct.
The belief in reincarnation was used by the high caste in India to control the low caste. The low caste were led to believe that if they accept their low status without protest they may get a chance to be reborn as a high caste.
Some people find it difficult to accept that, maybe life ends with death. They really want to keep on living in one form or another. For such people reincarnation is an attractive option.