after halflife, and counter-strike, STEAM was their bread and butter. Counter-strike's success was integral to everything because then you had other people use their halflife engine to make other successful mods, such as Team Fortress Classic, which they bought and later turned into a cash cow that is Team Fortress 2. Since people were making successful mods they got the idea of making the source engine and licensing it out to diff companies to make their games with. Steam was released around the time that the beta for the first counter-strike expansion and they built their success through the STEAM marketplace.
Something that pisses me off is companies like EA who used to release developer kits for battlefield no longer do that. You can't even imagine the amazing mods we're missing out on for Battlefield 3/4. They used to have star wars battlefield mods for BF2, and other interesting mods as well.