Google has relied on what it calls "signals" to understand consumer intent and target the right information like paid search listings. Essentially this is behavior like what you click on and where you have been. Facebook relies heavily on our profiles, what information we all share about ourselves and the connections we have made with other people to deliver relevant information (and advertising). For Google to deliver deeper value via more 'social' benefits, they have to get us all to create profiles like most social networks. +1 drives us to do that. In order for me to see what my Google "friends" have clicked on, we all have to have Google profiles. Many folks created those when they signed up for any of the services that Google offers from Gmail to GoogleDocs and more.
The success of +1 is tied to two things. Will tons of folks create Google profiles and/or start taking theirs seriously? And does the actual +1 functionality deliver enough value. (note: it caused me to go in and add a bit more to my profile...)
Not sure if Google's new "+1" button is more like "Like" or more like my Delicious account (which I will mourn the loss of when it goes away). Currently +1 doesn't allow tagging but that would be a natural next feature. I am trying it via Google Labs.