It was at the insistence of the actor. He wanted to look more like the iconic images of Bruce Wayne that he'd seen in earlier comic serials. The wig also served to age him up.
Ben Affleck had been immersing himself in the comic books and had seen an image of Bruce Wayne. "I need my hair to look like this," he informed his filmmakers, brandishing the comic book in question.
"I remember not being too sure," admits Deborah Snyder. Later, she and Snyder were in Michigan busily readying the shoot, with Affleck still in Los Angeles taking meetings with head-of- hair Chase Heard. A photo arrived of that hair. "It was perfect - he looked like Bruce Wayne." The producer was delighted to be proved wrong. "It was that moment: Oh, we gotta do this."
Truthfully, it's nothing drastic. Affleck wears a wig in the movie that makes concrete Wayne's age and experience. It instantly made him their Bruce/their Batman: older than ever before. Older, in fact, than Affleck (42).
"We are playing him 45 or 46," explains Snyder. "He has been Batman for 20 years. All the history is there. Was there a Robin at one time? Possibly."
Empire UK: September 2015 - They are Legend