What is does (apparently) is to pass up targeted ads (in text format) at the bottom of the emails.
I won't post the link i found... it's in french.
Nevertheless, 1Gb for free is pretty nice, hopefully it'll push the other large webmail companies (hotmail, imail, yahoo, excite...) to up their limits.
Oh, and Rip... a cookie is just plain text, what it can probably do is store the references to the products you're interested in & set the related adds to those products.