...and sometimes I use -no_emissive_light, which removes all ambient light in the game.
Actually, -no_emissive_light simply turns off emissive light.

Emissive light is a general glow that objects give off (well, certain objects anyway). It's not the least bit realistic for a game like this, but it prevents everything from getting pitch black. Even if there is no ambient light the emissive light should still show objects to a basic degree. But without emissive light you will only see something if there is light actually shining on it, either from the sun(s) or weapons/explosions. Turning it off really adds to the atmosphere, but it's on by default as an aid to users.
Now, using -ambient_factor 0 will deffinitely disable ambient light. At least down to 2%, which is the lowest possible value the game allows. At that level you are relying largely on emissive light to actually see anything in general. You'll mostly just be able to see things which are lit up by weapons fire or explosions, or other 3rd-source light.