Cousins is probably best known in the baseball realm as the player that injured San Francisco Giants catcher Buster Posey in a home-plate collision in 2011. Other than that, he has made most of his living on the Marlins roster as a fourth/fifth outfielder known more for his glove than his bat.
He is a career .183/.231/.291 hitter in 188 plate appearances. As Jeff Sullivan of Lookout Landing notes, Cousins is joining a crowded Mariners outfield that features a whole load of players that can't really play outfield at the major-league level consistently. It's a low-cost gamble for the Mariners to see if Cousins is better than Mike Carp, Eric Thames, Carlos Peguero or Casper Wells.
All of these guys could have futures as starters, but more likely is that they don't. The Mariners had outfielders, but they needed to consolidate. They needed to find a legitimate everyday player.
The Mariners' 40-man roster currently sits at 37 players. Cousins has one more minor-league option left while Carp, Robinson and Wells are all out of options. If either of those three do not make the major-league roster at camp, they will be available to be claimed by another team.