I have the original three D&D books plus Greyhawk. Back when I was in my teens, I wanted copies of the supplements but couldn't afford them all as they were quite pricey. They're still pricey, but I'm earning more money now than when I was a bingo caller so I can afford them.
What's that? Do I need a supplement I'll never use for a version of a game I'll never again play?
Yes. Yes, I do need it.