In a big conference you need people who can turn one language into another in real time. This of course is something of an art, because the interpreter has to convey the meaning of a series of ideas that are still being formed.
It demands a high level of attention and alertness, with the interpreter conscious of all the the non-verbal aspects that are going on in the room. That’s why a booth is designed to give the interpreter a good visual access to the podium.
It’s also why the booth is designed for two people. Taking turns in 20 minute shifts it’s one interpreter on, and the other taking a short break.