+1 is just the latest iteration of Google's social networking attempts, so in a way they've always been working on it. As for why, well, Google is in the business of organizing data and having detail data about its users' social life is a goldmine. Although I'm not sure that it will improve the quality of their search engine, as many people have stated, it will most definitely tie users even more to Google and that's always a goal in itself.
I think it will help with some of the spam since users will generally not +1 a spam site. It will also add an extra attribute (social) to the search algorithm.