First of all, thanks very much for the question- evolutionary biology/neuroscience is very interesting. The frontal lobe is devoted to higher cognitive reasoning, so that includes decision making, goal-directed behavior, working memory (holding a thought in your head), and emotions. As for how they grow, I assume you mean evolutionarily. We’re not really sure how exactly they evolved (like so many other things), but we know that the size of the frontal lobe is up to 6x bigger in humans than it is in lesser monkeys or human ancestors. (Interestingly, there was a study that showed that the frontal lobes might be just as big in some monkeys, such as the great apes.) Take that in comparison to the brain being only about 3x bigger (in comparison to body size).
A lot of what “makes us human” is located and controlled in the frontal lobe. Homo sapiens were really the first organisms with such a large frontal lobe, and it is one of the defining points of our species. However, it seems like the most important part (of the brain) in causing us to “be human” might actually be the folds in the brain.
Our brains, even compared to other primates, have an extreme number of folds. This means that we can fit a lot more neurons in less space. The outermost layers of the brain are the cell bodies, so the more folds we have on the surface, the more neurons we have. However, we know that is not just the number of neurons/folds alone that make us intelligent. There was a project that genetically knocked out a gene that causes normal neuron death early in development. This created mice with many folds in the brain and many neurons that wouldn’t normally be there. Unfortunately, they were not “genius” mice- they were hardly viable (they did not live for long if at all). I can talk later about the “doogie” mice that did have seemingly superior intelligence compared to regular mice- and it was due to a different gene unrelated to brain folding. In other words, it seems like a lot of different factors add up to create Homo sapiens and define us a species. Large frontal lobes, many neurons, and a bunch of different genes effecting molecular systems developed over time to make us into what we are.