When would it be more useful to represent a proportional relationship with a graph rather than equation

It would be more useful to represent a proportional relationship with a graph rather than an equation when you need to compare two things visually. 

When you need to visually compare two things, that's when you would use a graph. (That is for qualitative data.) However, when you need quantitative data, nothing is better than using an equation. 

Hope this helps! ^-^