When the screensaver kicks in, it automatically requests data from spamming websites, putting a “load” on those servers. The screensavers are designed to NOT do a DOS (denial of service) attack on the servers, but to put a load on the servers, making the spammers at least pay more dough to host their crap.
The screensaver evens displays a cool graphic showing data requests hitting the various spam servers.
(To clarify, it’s hitting the websites HOSTING the spammed Viagra ads etc…its not hitting the servers that send the mail)
I highly recommend the screensaver (it’s for Mac and Windows last time I checked)