By linking his endorsement to a specific issue — climate — he does two things. First, he gives a pretty effective issue advertisement on the subject; Bloomberg would likely be able to get the cameras on him at any rate right now, but doing it in the context of a presidential endorsement is more effective than simply repeating what he's said in the past on the issue. Second, he's essentially lobbying the president on this issue.
Remember, votes don't speak for themselves; politicians must interpret what votes mean. In that, they tend to interpret through their own experiences on the campaign trail; that is, if they've been talking about an issue a lot, they tend to believe that those who voted them must have endorsed that position. Most of us can't do much about that, although perhaps more than we think — volunteer for a campaign, or if you have the means donate money, and you'll get at least someone's attention. But if you have a very large megaphone, you can do more, and that's what Bloomberg is accomplishing here.
Ben Smith is in the same ballpark:
It also turns the New York mayor, who had been searching for a next act, on the leading edge of an issue that Sandy had forced the media and political class, whose attention had wandered to the coal-heavy economies of the Midwest, to consider. Bloomberg's foundation has spent years building a climate initiative, and he has spent heavily on a push to shut down coal-fired plants. If Obama wins, the cause will finally have what it had lacked: a victory, and a political story to tell.