I believe the answers are:
The switch in narration increases the proximity of the narrator to the main character.
The reader can perceive information only through the filter of a single character
In a third person omniscient, the narrator is someone that know it all, from the past ,present, and future of every characters.
In Third-person limited, the narrator only know the past,present, and future of the main characters. So, this would increase the proximity that the narrator has upon the main character, but it made the narrator provide the information beside those that is known by the main character.