Do we really need to say it again? Batman and Superman form the polar ends of the DC superhero universe, leaving an open door for future characters to fill in the middle. And with Batman vs. Superman allegedly building right on top of the events of Man of Steel, a Justice League universe continuity is officially being established.
By the time Batman vs. Superman is done, the mutual respect between The World's Finest can act as the solid foundation upon which a Justice League team is built. Superman can be a little bit more vulnerable and more of the responsible boy scout that fans love, while Batman can be reintroduced to the world as a tactical genius badass who is able to stand alongside the most powerful being in the world, while also becoming more open-minded to the idea of friendship and partnership.
Where the DC universe goes after that is still an open debate, but in terms of building continuity on top of Man of Steel, Batman vs. Superman is in a better position than most other DC superhero movies would be.