Because he is slightly above league average in CS%, grades out positive on defensive metrics on both Fangraphs and Baseball Reference. Offensively, Snyder is nothing to write home about. Prior to 2012 he was a solid offensive producer. He was terrible in 2012, and barely played in the majors last year. But he can take a walk. Based on his career numbers and age, both Steamer and Oliver, two commonly cited projection services, estimate him at 84 wRC+ (runs per plate appearance where 100 is average; I note that the last cited article states the average catcher offensive production since 2011 is about 94). Then again, neither are Suzuki (Steamer 79, Oliver 84) or Buck (Steamer 88, Oliver 89).
The difference maker is pitch framing. Kurt Suzuki is bad at it and John Buck is terrible. As noted by another article:
After considering pitch framing, Kurt Suzuki and John Buck rank no better than replacement level.
and the author notes that if a team looking for a starting catcher does not sign one of the top four catchers (McCann, Pierzynski, Saltalamacchia, or Navarro), that team is "SOL," noting:
"SOL," of course, is Latin for "Now you have to sign Kurt Suzuki or John Buck." Latin is a nuanced language.
Snyder, it turns out, is pretty good at pitch framing.* He's not a long term answer, of course - he's 32, and will be 33 before the 2014 season starts. He is another option at backup catcher, and from what I can tell just as viable an option as Suzuki or Buck. The last cited article has him considerably better as an option than either (weighted for pitch framing, Snyder has averaged 0.4 fWAR for the past three years, 1.7 per 500 plate appearances; both Buck and Suzuki are at 0.0), but I'm not willing to go that far. But as viable an option, for a fraction of the cost. And he could well give Sandy Leon a chance to rediscover his bat if he's not yet ready, and keep the Nats from having to rely on Jhonatan Solano. And if he doesn't, the Nats can squirrel him away in the minors or cut him with very little cost.
*Some have criticized pitch framing as being partially a function of the catcher's pitching staff; the idea being that better pitchers will both throw more strikes and perhaps get more deference from umpires. Whatever the merits of that criticism, I note that over the last three years, Snyder's major league experience has been with the Pirates, Astros and Orioles. Not exactly the homes of the top pitching staffs in the league.