All of the reasons you've listed contribute to why plazas are often considered the heart of a town or city, but if we had to highlight a primary reason, it would be:
They are meeting places for the people of the town.
This aspect emphasizes the social function of plazas, where people gather, interact, and engage in community activities, making them central to the town's social life. However, the other points you mentioned are also significant and highlight the multifaceted role of plazas in urban settings.