No. 2 -- Affleck's Batman will counterbalance Cavill's Superman
In the recent press release announcing Affleck's casting as Batman, Snyder said Affleck, who's 11 years older than Cavill, can portray a man "who is older and wiser... and bears the scars of a seasoned crime fighter." Snyder may be on to something. Comic book fans know that Superman and Batman, while loyal allies, have a complicated relationship. Superman's optimistic heroism is often at odds with Batman's world-weary vigilantism (as Batman says in "Hush," the revered comic book series, "Even more than the Kryptonite, [Superman/Clark Kent's] got one big weakness: Deep down, Clark's essentially a good person. And deep down, I'm not."). If Affleck helps bring that tension to the Batman/Superman movie, it could be far more interesting than "Man of Steel" proved to be.
No. 1 -- It's 100% guaranteed that he'll be a better Batman than his friend George Clooney
Heck, even Clooney would agree with this one.