Many mods are interesting because the modder solved a personal problem of his. And some mods do end having a "studio" backing... For example the Network Addon

Many mods are interesting because the modder solved a personal problem of his. And some mods do end having a "studio" backing... For example the Network Addon Mod for SimCity 4, it started with several individual modders solving their own problems with the game, and now it has a official "NAM Team" that act as a professional team (they give release dates, have deadlines, make their own installer, etc...)