Named, reusable values (a length, an angle, a number, a yes or no) that live at the project level rather than inside a single family, and can be used to drive dimensions, constraints, and formulas anywhere in the model. Setting a "Standard Corridor Width" as a Global Parameter and tying every corridor dimension to it means changing the office standard once updates every corridor in the project instantly, instead of hunting down each one manually.

They are a lightweight entry point into the same "define it once, drive everything from it" thinking that parametric modeling and shared parameters are built on, without needing a Dynamo graph to set up.