A Google Profile is an online calling card. A Profile links together your online presences of choice (ie. twitter, your blog) into one coherent web-identity. This can help you clean up your web-presence for college and job interviews, and it can help you start to take control of your own Google search results.
Although there are many online calling card services out there (ie. LinkedIn), Google Profile is the simplest and most viewer-friendly. I feel comfortable adding my Google Profile to the signature of my e-mails, even when addressed to friends and acquaintances, not just "networking contacts."
Plus, I prefer a life in which the line between friends and "networking contacts" can be blurry, anyway.