Matrix operators are a set of Dependency Graph (DG) nodes that can be chained together to do complex math operations to use as input to the Maya matrix-driven transform nodes.
The addition of a matrix input on transform nodes opens up new methods to drive a transform that differ from the traditional Maya workflows. Using Matrix operators means you don't have to extraneous DG and directed acyclic graph (DAG) nodes with multiple connections that are often time-consuming to modify, edit, and compute. The Matrix nodes are built to be procedural. For example, both the proximityPin and uvPin Coordinate system can be edited after creation time to accommodate new inputs.
When you created constraints previously, you often had to delete and recreate a constraint just to make a modification. The procedural nature of these nodes means you can modify settings while you work. The Attribute Editor exposes many attributes, providing granular control over how the system operates.
The proximityPin node outputs positions by calculating the closest point on a surface to a given world position. See Proximity Pin.