So taking Doom as an example, Doom now runs on just about every computer you can imagine, from handhelds to pregnancy tests. However, just having Doom run everywhere is only part of it, the various "ports" of Doom have also enhanced the original engine, adding new features like mouselook or even entirely new rendering engines.

Because it's all open source, all of these changes and enhancements can be used by different ports and they all benefit from it.

Correct me if I'm wrong, but weren't the original Half-Life based on this engine?

So, Counter-Strike wouldn't have existed in this form, and DOTA 2 wouldn't have been made in the engine it was.

Conversely, Warcraft 3 also had a modding scene where DOTA sprung from.

So, probably a lot of Valve's games wouldn't have seen the light of day, and they wouldn't have had the capital to make Steam.