Does not eliminate room noise. If you have worked sound on a film or TV set I'm sure you are familiar with capturing room noise on a quiet set to mix in during post.
Yeah, you're right. Sound guys always pick up room tone before or after the shoot. I was under the impression shotgun was used to eliminate/avoid sound sources that are not desirable. From my, limited sound experience, I've seen sound guys use it like that on my sets. For example, there's some noise from certain direction, but I am told not to worry about it since it won't be picked up with shotgun. Lo and behold, it wasn't there in the source material. It's probably due to many factors I'm not aware of though (sound reflecting of stuff etc.). They do turn off all the A/C's, fridges, and everything that's humming and buzzing though.
The reason shotgun microphones are used is because the polar pattern[1] is highly directionally. Basically the amount of sound that a shotgun type microphone picks up is very strong in one direction and less in another. This reduces peripheral sound being recorded at the same level as your target sound.
