What is a reason why plazas are often considered the heart of a town or city

because plazas refer to more open areas that people go too, for example Times Square is like a plaza because of how crowed it is.

Plazas are usually rather called: Foundational Plazas (Plazas fundacionales in spanish) Because there were the first thing to be foundaited in the city before this was one, therefore everything starts to be built around the plazar just as; Governamental entities like the major's office, the police station, and the church.

So making it short, the plaza is the first thing to be built after the church, and everything around it makes part of the most important entities of the city (That used to be a town)