Because lots of big companies are aware of it and do use it. Plus checks for it are written into Spamassassin. So my point is, lots of anti-spam services chec

Because lots of big companies are aware of it and do use it. Plus checks for it are written into Spamassassin. So my point is, lots of anti-spam services check it, but few people sending mail make an attempt to get their server listed in there properly. Mail Servers get listed in there automatically, but if you go in there and vouch for your mailservers properly you're get a more trusted ranking, which translates into a higher anti-spam ranking when checks are done.