Although, when it comes to US services, Brussels and Berlin are - sort of - secondary airports...
Brussels has no AA service at all, UA to EWR, IAH and ORD and DL to JFK. And SN serves, IIRC, just JFK. And BER is even worse, with just a singe year round lfight by UA to EWR. DL to JFK and AA to PHL are seasonal and LH has cancelled its service.
At both airports, Primera sort of fills a void created by the disappearnace of TATL services by Jet Airways (BRU) and airberlin (TXL).