Earlier quoted context omitted.
One point raised on the article is that they will have a better idea of market and salary range than you will (or they should!). So if you give them your desired salary it might be out of their range (too high or too low). Also, especially at the beginning of the hiring process, you don't know everything about the company, so how can you have a valid desired salary? Also, your value to them as a possible employee inc…
Why wouldn't you have an idea about market range?
Again, it's primarily a negotiation tactic.