What does an underground electrical pull box do?

An underground electrical pull box provides another access point in long runs of cable to make it easier to pull the conductors from one end of the run to the other. The crew can only slap so much silicone lube on the conductors and the the conductors can only have a limited amount of force applied on them to pull them through the conduit. If a long run is broken up somewhere in the middle, the crew can pull to that point where the pull box is located, and then "start a new pull" from the box to the end of the run. It's more work, but it protects the conductors and the job ends up "meeting code" and passing inspection.