Alfonso Soriano is old (37), expensive (owed $18 million both this year and next) and only marginally valuable (a 0.7 bWAR this year). To a rebuilding team like the Cubs, he was an albatross. To an old, expensive team just 2 1/2 games out of the wild-card race like the Yankees, however, that marginal value might just be worth the cost, particularly given that New York has dropped well below replacement level at Soriano's primary position in leftfield.
Soriano began his career as a Yankee, left on good terms in the February 2004 trade for Alex Rodriguez, and, as a 10-and-5 player with the right to reject any trade, was open to returning to New York. That, along with the brutal performance of the Yankees leftfielders and righthanded hitters, made a deal sending Soriano, who addresses both of those problems, back to the Bronx a no-brainer. Indeed, that deal is now official, with the Cubs due to receive Class-A right-hander Corey Black and the Yankees picking up $6.8 million of the money owed to Soriano over the remainder of this year and next, when his eight-year, $136 million contract finally expires.
If that return seems underwhelming to Cubs fans, let me put it this way: The Yankees gave Chicago $6.8 million and a pitching prospect for Alfonso Soriano. Yes, the Cubs still owe Soriano almost $18 million, but they weren't going to do any better anywhere else. In fact, given Soriano's veto (he nixed a trade to the Giants a year ago), there was little to no chance of Chicago finding any other team that would both need Soriano and to which he would be willing to go.
And the Yankees do need Soriano. Their leftfielders have hit .223/.268/.330 this season. Their righthanded hitters have hit .221/.283/.311. Vernon Wells, who has hit .240/.288/.366 on the season and .192/.232/.231 since hitting his last home run on May 15, leads the Yankees in at-bats in both categories.
Soriano has hit .254/.287/.467 this season and arrives in the Bronx hot. In 21 games since June 28 he has hit .286/.330/.714 with 10 home runs, giving him 17 on the season. Even with that miserable .287 on-base percentage, which bests only 13 qualified hitters this season, Soriano should be a huge boost to the Yankees, whose team OPS in those splits (.598 in leftfield, .594 by righthanders) are among the worst you can find in Baseball-Reference's searchable Play Index (though such historical split info is far from complete).