One of the first things that gets done on a new job site is any underground work needs to be installed. On a lot of jobs electricians and plumbers need to run conduit underground to specific locations within a slab. Sometimes this is done before the slab is even formed up and other times it is done after the Beams are dug. Either way during this time you are getting raceways underground set up to the locations that you need them after the slab is poured.

And example of this would be if you have a specific place in the structure that a panel needs to be located, And the only Way to get the conductors to the panel from the service is to go underground and pop-up into an interior wall in the structure. In this case you would have to dig a trench and put the conduit up in it, and have that conduit stub up through the slab before they ever pour the concrete. Then once they pour the concrete your conduit is still in place and when the building is framed up you can come back later and pull the conductors into the panel once you get it set in place.