The World Heavyweight Title only exists for three reasons
1.) To placate the guys who can't fit into/aren't good enough for the real main event, but WWE still wants them to be "stars". Instead of having to create stars, the WHC allows them to manufacture stars.
2.) In order to give wrestlers a "big title win" without necessarily having to go all the way with them.
3.) Having a second World Title allows them to keep their golden boys like Punk/Cena in the WWE Title picture because everyone else has another "world title" to go after.
A second title is also somewhat important for house shows. That way, if one world champ is otherwise occupied/night off or it's a single-branded card, the audience always gets a "world title match." Of course, they're dopes for thinking there is the slightest chance the title will change hands on said show, but so be it.