This saying might sound obvious, but as a rule of thought it can be very powerful. Many products will market themselves as green - and that may be entirely true1 - but there will always be some waste associated with the production of a product. Forcing yourself to reevaluate the need to replace something you already own even if the replacement is "green", will lead to lesser resource waste.
That said, a counter-argument can be made: Supporting companies that try to change the market, will aid in making that happen. Vote with your money, as they say. is a great example of a company worth supporting, though they actually implore their customers to do just what this note is about.
See the opposite: Greenwashing