My guess would be for analytics, so it knows how its own service is being used and who is accessing websites through it. It comes with a convenient feature that bad URLs can be taken down on its site.
Genuinely curious: How does it take down bad URLs? And, by "bad", are we talking about 404s?
"Bad" as in malware, phishing, etc. When they recognize that a URL is bad, they can simply stop redirecting to that URL.