If we had one central market, with a Walrasian auctioneer, where all goods are traded simultaneously for each other, none of this would matter. But we don't; so it does.
Partial equilibrium theorists don't need to know this stuff. People who believe prices are always at market-clearing levels don't need to know this stuff. Everyone else needs to know this stuff.
The world needs one person to prove existence of Walrasian general equilibrium, and a second person to check the first person did it right. Everyone else should learn this stuff instead.
If you are an old macroeconomist, or maybe French, you probably know this stuff. Otherwise, you probably don't. Because it all got swept away in the New Classical revolution. And then got swept under the carpet in New Keynesian "cashless" models (which aren't).
1. Let's count the number of markets.
Economics is supposed to be about things like goods getting traded in markets. So the least we can do is count the number of markets right.