Related to that is the ability to watch games using the game-client too.
This used to be a promoted feature in CS, with "HLTV/GOTV", but sadly disappeared when they moved to CS2.
Spectating in-client is such as powerful way to learn what people are doing that you can't always see even from a recording from their perspective.
Also allowed to watch games _live_! Long before streaming videos was a reality.
Ah, the good old days of watching live competition of quake through the game itself, chatting with others basically through the game console.
Pretty cool system.
This absolutely still exists - I have a library for reading Source 2 (CS2, Deadlock etc) demo files and streams (HTTP ones like CSTV).
> Related to that is the ability to watch games using the game-client too.
Halo 3's in-engine replay system was the high water mark of gaming for me.