Because they'd win the division. If they lose to us and San Fran to finish 14-3 and we win out here are the tiebreakers:
1) Head to head would be a split
2) Divisional record would be a split as we both would have one loss in the NFCE
3) Common games would also be a split as they have two losses in common games (49ers and Jets) and we'd have two losses in common games (Cards, 49ers)
4) Conference record - here's where we lose. We'd have two non-divisional conference losses (Cards, 49ers), they'd have only one (49ers)
So for all intents and purposes, Dallas needs to likely finish a game clear of the Eagles or have them finish with the same record but lose Giants giving us the divisional tiebreaker.