Board Thread:Questions and Answers/@comment-3431748-20160409212027/@comment-6369172-20160409213532

It's an artifact of publishing history. Way back before there was a Batman character, there was Detective Comics. The character was way more popular than many competing features, so Batman got his own magazine, named after himself, not long later, and it continued to sell real well forever afterward. So both titles stuck around.

Shorter: more titles ==> more sales.

Now that situation sometimes DOES afford the opportunity to run parallel story arcs, or to run one arc across both titles, and there's plenty of times when those things were done, but really, truly, under all that, it's the sales. More is better.