The most fitting reason why plazas are often considered the heart of a town or city is:
They are meeting places for the people of the town.
Plazas serve as central gathering spots where community events, social interaction, and public life flourish, making them a focal point for civic engagement and community connection. While the other options also highlight important aspects of plazas, the role as a meeting place is fundamental to their identity as the "heart" of a town or city.