It just points out the subjective nature of the business. People throw out terms like “good books” and “bad books,” but the only truly objective measurement we can assess is how well the book sold. I'm not trying to say that that good writing always sells well (or the reverse). I just mean “good” is a very subjective assessment. In his acceptance speech for winning the Hugo for the novel Forever Peace Joe Haldeman thanked his agent for taking his first novel The Forever War to that 18th editor because the first 17 had rejected it. The book won the Hugo and the Nebula and is still in print more than 30 years later! I am quite sure those 17 rueful editors didn’t think it was a bad book. They just didn’t think it would sell.
A huge part of an editor’s job is to decide what books to buy (and then publish). Most editors become editors because they love books, but publishing is a business and they have to do their best to pick what will sell. This puts us writers in the position of trying to write the “best” book we can and then hope that someone will see it as a book that other people would buy.