As was the case when Mark Teixeira was signed last year - although obviously to a far lesser extent - Brian Cashman decided that Chan Ho Park was worth lobbying the ownership to spend extra for. Park rejected a $3.25M offer to stay with the Phillies midway through the offseason and watched his price fall steadily until, apparently, the Yankees couldn't afford not to sign him.
Park began 2009 as a starter for the Phillies, struggled and was ultimately bumped to the bullpen after a particularly dismal 1 1/3 inning, 5 run effort against the Nationals in mid-May. Once transitioned to the 'pen, he was extremely useful for the Phils, pitching more than one inning in 13 of his 38 appearances and compiling a 2.52 ERA over 50 innings in relief. Park blew Game 2 of the NLCS against the Dodgers but was otherwise very effective in the postseason as well, particularly against the Yanks in the World Series (3.1 IP, 0 ER).
Prior to the move, it seemed like the Yankees were essentially set in the bullpen with Rivera, Hughes/Chamberlain, Robertson, Aceves, Marte, and some combination of Gaudin/Mitre/Logan/Melancon. Given the Major League deal to Park, it's likely that he makes the team out of Spring Training. Consequently, the Yanks may decide to trade Gaudin and his $2.95M salary, as he could be more valuable to a team in need of a 5th starter than one who would employ him primarily as a mop-up man. They could also decide to stash the loser of the much ballyhooed Hughes-Chamberlain Battle in Scranton to start the season, although Joel Sherman doesn't think that is very likely.
Of course, the Yankees are going to catch some flak for this because of the emphasis they've continued to place on the budget. But Brain Cashman isn't a politician. He doesn't need to get re-elected or appease his constituents, so it doesn't matter that he deviated slightly from a previously stated plan (or two). In the end, it's not about sticking to the stated goals; it's about making the team better.
All told, the signing does seem likely to make the Yankees better. During an offseason in which many teams were more than willing to pay good relief pitchers far more than they deserved, Cashman and Co. found a relative bargain. You can question whether or not the signing was really necessary - or worth exceeding the budget for - given that the bullpen was already in good shape, but it's hard to argue that this isn't a good value for the Yanks.