Raising a family, as in creating offspring, providing for that offspring, ensuring a high quality of life, and preparing the offspring to go on and do the same. The traditional model provides the most well rounded way to do this.

Males have (arguably) higher earning potential overall in our society as it stands. This is for a few different reasons, and that is an entirely different discussion. A man's strength in this system is, primarily, generating income. There is still a role the man plays in raising the child, and their emotional development (which is important for the child in their ability to fit in with society and be successful later in life), but their main duty is as a provider.

Females are (arguably) better suited for raising a child. From conception, a child relies entirely on their mother for survival. As the child goes through life, they become less and less reliant on their mother, but this is a direct result of the mother's ability to properly raise the child to develop the strength to become independent. A mother feeds, clothes, bathes the child, and endures the home is a fit environment for it. This is where the woman's strength primarily lies.

These roles are important no matter who fills them, in my opinion. However, I also believe that humanity has developed with certain characteristics that allow for one gender to be better at some rolls, and not as fit for other rolls. I know that there are certainly exceptions, but as a whole, that is just how it is.

As for this improving humanity, people playing into their strengths makes themselves stronger. Strong individual families all living and working together makes for a stronger society as a whole. Strong societies coexisting on the same planet make a stronger and better humanity.