Why do cats have more than one kitten?

From the point of view of genetics, a "successful" animal is one which passes on its genes to future generations. Some scientists think that it is the genes which impel creatures to breed.

Cats have short lifespans and (in the wild) there are many predators, so a successful 'strategy' for breeding is to have several offspring at one time... this increases the chances that the parent's genes will make it to a third generation - it's not enough to reproduce, the offspring must also reproduce...

Animals don't actually have strategies thought out, it is just that those animals which do have multiple births will be the ones whose genes are passed on.... genes that will ensure that the next generations will also have multiple births.

Some species favour single-birth reproduction, as the parent or parents can then devote more resources - time and food - to trying to ensure the survival of their single offspring.