Valve originally was a game engine company just like Epic and Id, and they still are but have broaden into a platform and publisher. All three companies built games to show off the capabilities of their engines. Valve is unique for also caring about storytelling and rich game mechanics compared to Epic and Id, at least at the time. HL1 was as much about being a tech demo as it was about being a great game.
Not true. GoldSrc wasn't ever really sold. They just acquired mods built on the engine (which was source available) to sell themselves (like counter strike or team fortress)
