Two dryer vent connections in the laundry room
I believe the second capped dryer vent is a cleanout particularly if it is located in an area where it would NOT be covered by the dryer.  My guess is that the capped vent is beside the dryer and exposed while the vent opening is behind where the dryer would go.  The push today is to use fixed steel piping not the flexible junk since the plastic flex junk has been known to catch on fire when plugged up.  That means dragging the dryer out to get it out of the way is a major production involving contortions and likely at least once dropping a screwdriver behind the dryer while fumbling with the clamps.  It would be much easier to just remove a cap and stuff your vent pipe vacuum up into it and leave the dryer be.
But since this is a brand new house why don't you just ask the builder?