We all know the "head the family" is not just referring to a floating human head, but instead the entire person who holds the power in a family unit.
It's a shorthand way of giving us something small, specific, and often visible to refer to, especially when the whole concept is a more abstract.
We refer to a collection of businessmen as suits.
For politicians who run a national government we use the physical places they meet like Parliament, or Number 10, or the White House.
To tell someone we like their car we say that's a nice set of wheels.
We might ask for a hand carrying in the groceries, when we really need a whole human to come help out.
We pick physical symbols like bits of clothing, buildings, limbs, or locations to give us a tangible reference point for a less tangible group of people or an idea.
Synecdoche is especially important in creating Visual Explanations and Visual Metaphors, as it allows us to reference non-physical concepts in visible, physical symbols we can draw.
It's an offshoot of Metonymy - when we refer to one concept using a different but related symbol.