If, say Slayer or Lamb Of God are playing in/around Chicago around the date of the Ozzfest stop there...the idea seems to be then that they would be invited to play the Ozzfest show.
The obvious drawback to a band doing that would be possibly less people at their headlining show. But, one could assume that the fans of that particular act would see their headlining show anyway as it would be a longer set & tickets aren't exactly thru the roof for most metal acts anyway.
The positives would then be to play in front of a larger crowd, 'give something back', sell alot more merchandise than they normally would at their 5000 seat venue.