Why might it be better to seperate substances by a physical changes rather than by chemical changes?

Answer 1

A physical seperation enables you to get the individual parts with the properties you desire. A chemical change will create an entirely new substance.

Imagine you wanted to seperate gold from sand. You would want to isolate the gold and get rid of the sand. So a physical seperation such as settling and filtration would be good. If you make a chemical change, you would lose the gold as a metal and get a gold salt complex instead.