He was signed as their starting LF, I believe.
And while the Nats overpay, it's not ludicrous. All their signings - Marquis included - were within the going rate, but the Nats will always have to lean towards the higher end. The team has not outbid itself, though.
For instance, Marquis is also getting $15 million (albeit for two years), and is a solid middle-of-the-rotation guy that any ballclub would take. I would gladly give that guy $7.5 million a year, versus $5 million a year for a 32-year-old outfielder with a spotty track record.
(Byrd, by the way, was traded by the Nationals because he tanked here.)