According to Diamond, many of the inventions that have most shaped modern society were not invented to fill a demand. Many, in fact, were put to uses far different than what their inventors envisioned. This is true, he says, of "most of the major technological breakthroughs of modern times . . . ranging from the airplane and automobile, through the internal combustion engine and electric light bulb, to the phonograph and transistor."
When the automobile was invented, people got around quite well in the same way they had for many thousands of years—on foot or horseback. In addition, the invention of locomotives, electric streetcars, and other vehicles seemed more than satisfactory for getting people around. The automobile changed people's lives in ways nobody could have predicted when it was first developed. And after it was invented, people wanted one, to the point that it became a necessity then and now in modern life. Diamond argues that "many or most inventions were developed by curiosity or a love of tinkering" rather than by heroic geniuses (like Thomas Edison, James Watt, Alexander Graham Bell, etc.) who set out to better society. They also tend to be improvements on already existing inventions rather than the products of one person's brain. This discussion of invention is part of a broader explanation for why technologies proceeded and developed differently in some societies and geographical locations than others.
What Diamond says that really goes against the "heroic" model of invention is that individual inventors are not really all that important. He argues two things. First, he says that by far most famous inventors did not actually invent something completely new. Instead, they tinkered with something old and improved it. Second, he argues that if the famous inventors hadn't come around, someone else would have made the same breakthrough. So he's saying that invention is a process that is done by building in small steps, not by some genius having an inspiration and creating some finished product right on the spot.
Diamond's argument about the "mother of necessity" also takes some of the shine off of individual inventors. He is saying that these are not people who invent something because they see a need. They are not out to help humanity by solving some problem. Instead, they are just playing around with technology, trying to devise something new just for fun, almost. That takes away from their heroic image because it means that their discoveries are way more random and way less heroic.