Andrew Bailey knew for a long time that the Boston Red Sox were after him, so he's probably been preparing to be a member of the team for quite some time. Bailey knew Boston probably wanted a closer, and that they'd be willing to pay up for him and give him a chance to star in a big market. Based on how he sounds, he seems to be relishing the opportunity. Jane Lee has more on the situation.
"I love the Bay Area, but if you’re going to get traded, I can’t think of a better situation for myself and my family to be in. It’s a big market, big team, and I’m excited about the opportunity to win over there, especially knowing Bobby V."
The Sox are a team that have struggled recently to have successful moments in the postseason, and some have that has come down to shaky closer play. With Bailey, the Red Sox are hoping to have a steady hand in the bullpen they can rely on turning the ball to in the ninth inning.
The Oakland Athletics will undoubtedly miss Bailey, as they now have to again sort through their pitching staff and figure out if they have someone they trust to close baseball games. It's hard to see anyone that fits the profile on the team at the moment.