EWR-IAD is insignificant. For O&D, downtown Newark to downtown DC is 2:30 on the Acela. Who would schlep out to EWR, fly to IAD and schlep back into DC? But, for anything else, you can fly xEWR or xIAD, so why fly between the two other than in extraordinary situations.

For US, the LGA-PHL and DCA-PHL are almost nil O&D, again because it's silly to fly O&D. But, there are those who prefer to fly DCA-PHL-TATL rather than endure the nightmare of IAD in order to fly nonstop on any other *A carrier. Sometimes also cheaper premium fares that way.