Antenna software files commonly use the cartesian coordinate system to refer to points in space. X and Y are in the horizontal plane and Z is height with numerical coordinate values to describe position, length and element diameter etc. Unless your text editor provides a search-and-replace function, it can be tiresome making changes and difficult to spot errors.
Seen above two antenna file examples, to the left, purely numerical format and to the right symbolic dimensions/expressions.
By using symbolic dimensions geometry changes become easy. As an example, to change the reflector size, simply edit the single line that defines ref. For certain antenna geometries such as describing angles, it's very easy using expressions in symbol definitions particularly when making changes.