Personally for Arrieta, a good showing is key for several reasons. He has struggled this year at Camden Yards (2-5, 6.85 ERA in nine starts). And though he pitched well in his lone game against the Nats this season (a seven-inning, one-run no decision on May 18), he is 0-1 with a 7.20 ERA in three games versus them in his career.
But more than that, Arrieta can't afford a rough outing right now. Chris Tillman and Zach Britton are a phone call away at Triple-A Nofolk. And he could always get pushed back to the bullpen again if he's not producing.