Just one reason I came to this idea, is most of typography styles in my files have line height & letter spacing in percentage:
It is more convenient setting percentage for these, instead of individual one like 21 for 14, 24 for 16, etc.
But for example, when 150% is not suitable in project anymore, and need to be changed, I have to change it in all styles, one-by-one. I need some places or tools or variables to change them all at once.
So your Number variable, except for Text content and Font weight, supports all properties in pixels. What if we also had it in percentage by typing an extra % character and look like this?
Then remove the % by clicking on it at uncheck the menu?