but could have some special meaning to the target site.
Stressing the "could have special meaning". One could write a script to
spam a mail list or scan a SMTP server and generating these addresses. Some
HTTP server on the site can decode the info back into your mail address so
one would build a list of valid addresses.
Also, any info can be encoded there so when you visit the site, info would
be submitted. It cannot get hold of -say- your list of ports because it
doesn't run code on the local machine but it can have all the abilities of a
cookie. Actually, it's equivalent to a "submit" with no input from a user.
The only use that comes to my mind now is validating addresses.
I also had some trouble with a pre-padded "C4A" or similar related to a
worm that sent mail to address book. It pre-padded this string for some
reason. I see no use for such padding in the attacker view but it did help
me re-route all addresses beginning with that string to a phantom account.