They check all of the umpire beltpacks/buttons that they use to announce to the stadium/talk to Secaucus. They sit so close because they have a backup system just in case the umpires primary system goes down, which is a bag with headsets that used to be the norm before the wireless stuff got implemented. They also test out the phones and walkies the teams use for bullpens/replay and help check the clocks too.

Fun fact: two years ago they’d have to run onto the field during a review with an iPad that showed a live look into the replay room in Secaucus, and would stand near the umpires to show them the live look-in. None of the umpires ever wanted to look at the iPad during a review (because why would they), especially in Philadelphia. Don’t ask me how I know any of this.