One small example is how holding down Option/Alt modifies behavior in various ways throughout macOS.

Often it functions as a “do this for everything” modifier. So for example, option-clicking the minimize traffic light minimizes all windows from the application the window belongs to, and option-clicking a disclosure triangle in a nested list expands or contracts all child nodes.

There’s tons of little things like that which might sound silly but become significant time and sanity savers after making a habit of using them.