Simple drafting programs just let you draw like you’d use a pencil. But modern programs use parametric models to provide several benefits. One is that you can use parameters to change parts of your design and other parts will alter to take account of your changes. The other advantage is you can use one model for many similar but different designs. [Brodie Fairhall] has a nice video about how to use parameters in FreeCAD.
The nice thing about parameters is they don’t have to be just constants. You can put in formulae as well. For example, you could define one line as being twice as big as another line. You provide various constraints and parameters and FreeCAD works out the shape for you, keeping all the constraints and formulae satisfied.
[Brodie] shows how to use spreadsheets to manage complex parameters in big projects, which is pretty handy. There’s an upcoming feature that will allow you to group parameters into sets. As an example, you can see a model of a NEMA stepper motor that can change from a NEMA 17 to another size by just selecting different parameters from a configuration table.