Interchangeable parts allow mass production of commodity products. Commodities, despite what you may have heard, are seriously cool. They're great for the economy, because they're cheap and plentiful. The end result of any truly mature market niche is commoditization. The cost of production goes way down, and the full power of capitalism is unleashed as multiple vendors compete to provide a better deal on the same goods and services in order to attract customers.
Commodities are the lifeblood of capitalism.
The printing press commoditized printed pages, which changed the balance of power in Europe. The Catholic Church had a monopoly on written information, due to the monks it could employ copying manuscripts by hand. It made doubly sure to protect that monopoly by writing books in its own proprietary language, Latin, which nobody else used anymore. The Church was so powerful it crowned kings throughout Europe, and that unrivaled power lasted for over a thousand years. When cheap and plentiful copies of the Church's secret writings started turning up in local languages, their power base started to erode.