If memory serves me correct...that is what they are for; customs. Customs is on the lower level of the terminal -- so you would need to direct the passengers downstairs and not out into the general waiting area.
Ouboy is correct. Immediately after clearing the jet bridge, there is a staircase, which will take you downstairs to customs. Next to the staircase, there is a doorway, which opens to the central concourse.
One of the jetbridges is used for A320/B757 aircraft, while the other is used for the door right before the wings and the engines. I spent 3 weeks at DTW for Northwest training, I always found this odd...
As previously stated, they have a staircase leading to the lower level for customs. IAH, MAN, and FRA, along with many other intl. airports have similar setups. Basically, when you area boarding a departing flight, you go down a ramp or stairs to the jetbridge. When you arrive, the ramp/stairs going back up to the main terminal are blocked off, and you take another door to customs.
Actually there is a really long escalator that takes you down to the lower level for immigration, international baggage and customs. Also inside each one of those buildings is an elevator for the handicapped international passengers.
The dual gates are actually designed to handle an aircraft like the A380 if Northwest ever gets it or another international carrier flies it eventually to DTW. The gates I hear are designed to raise to a higher level.