It is more important to know the domain of the function to be able to determine the corresponding output set so the function can be graphed. To start with the domain is very useful because every domain element has a corresponding unique output element.
Suppose you started with the range element of 9. The input set for an output of 9 is {-3,3}. This makes it hard to match up the elements. This example highlights why it is important to start with the domain rather than the range.
