I'm nowhere near Linux guru status, but it seems to me to be a security warning. If sendmail has root privilages, any exploit which is used against it could run on your system with root privilages. If sendmail is running as, say, nobody, than it will only have the powers that nobody does, and thus will be unable to compromise your system more severely.

I may, of course, be completely mistaken, in which case I'm sure someone more knowledgable will LART me.