[mod delete - no longer needed with title change] UA is doing two things:

1) Expanding DCA-EWR flights to "nearly hourly"
2) Adding IAD-LGA flights.

There is not a new DCA-LGA flight.

Really neither is a benefit for DC O/D traffic - seems like it's geared to connections xxx-IAD-LGA and DCA-EWR-xxx. (Added: Though increased frequency a benefit to DC-based travelers using these routes)