Why do normal weight women or obese women need to lay down extra fat stores for pregnancy?

I thought women have the fat they do BECAUSE it’s for pregnancy and those stores are laid down during puberty. Why would you need more? It doesn’t make sense to me. Plus, so many women have a hard time losing the weight they gain. And if they weight was for the baby and presumably the energy is going toward the baby, why would a woman retain any fat at all? I’m pretty sure in our hunter gatherer days women weren’t fattening themselves up to carry a baby.