Two words: Chris Weidman.
Chris Weidman and Michael Bisping need to fight one another to determine who the true No. 1 contender to the middleweight title is.
Aside from that basic fact, there is another reason that the fight needs to take place.
Chris Weidman is the future of the middleweight division and Bisping is, or will be, the past.
Weidman is 28 years old, Bisping is 33. Weidman is a dynamic mixed martial artist with an insane wrestling and grappling pedigree, and Bisping, while talented, isn't nearly as well-rounded and doesn't have nearly as much a chance at beating Silva.
Despite the disparity in their talents, Weidman is a lesser-known fighter.
What he needs is to be established as a star and as a legitimate threat at middleweight. To use a pro-wrestling term, he needs to be put over by somebody—somebody bigger than the likes of Mark Munoz, Alessio Sakara and others.
Bisping is therefore the perfect candidate to fight Weidman.
Bisping is a well-known middleweight fighter, he's at the highest echelon of the division, and is 5-1 in his last six.
This thunder, this name value, this "fame" needs to rub off on Weidman, and that can only be done by having both men meet in the Octagon for the chance to face Anderson Silva.
If Bisping wins, the UFC gets an established star to fight Silva for the title while still establishing Weidman as a fighter who belongs at the top of the middleweight division (and, more importantly, at the top of fight cards).
If Weidman wins, they get a new "next big thing" on whom to try and build the weight class, the same way they're building light heavyweight on Jon Jones.
Either way, the UFC wins from having Bisping vs. Weidman—the fight needs to happen and it needs to happen soon.