They didn’t add all that many units, two per race. But they did have a great impact on the game (mostly).
Also, new campaigns for each race was awesome. The level editor not only brought many fun custom maps (I still think about that weird 300 map I played when I was 16), but ensured longevity of the game until this day by enabling new maps to be played in regular games.

I miss getting all this stuff with a game or expansion too.