What was really revolutionary, IMO, about Half-Life was how moddable it was, and how Valve supported the modding community and often acquihired good modders.

Oh, and when they purchased the Counter-Strike IP, and started selling it shrinkwrapped, if you owned Half-Life, you just kept using the mod as you already were

In these modern times, I suspect that would be monetised somehow now. Upgrade to a premium user to get golden lootboxes, likely.

But yeah, I knew people who bought Half-Life just to play CS when it was early beta.

Likewise, less famous, but still being played today, Day of Defeat - I was hooked as the experience of "Yeah the rifles can kill you in one shot, and grenades will kill you, it's not CS dude. Oh and press B if that icon comes up, it means you're bleeding"

(Still unsure how I feel about them taking bleeding out.)