After Successfully logged in this url gets all old cookiees which stored in browser in request header and verify with server and update it if any changes.
generally this cookie always changed during each request
1. __utma,
2. __utmb,
3. __utmc,
4. __utmz.
, чтобы увидеть живую демонстрацию изменения cookie, используйте аддон Firecookie в Firefox.