This version of Batman seems to only appear when something huge's at stake. He's in a semi-retired state, you may say, which is why he wasn't relevant prior to the events of BvS. In BvS, he made his appearance because he perceived Superman as a threat. In Justice League, he believes himself to be useful because, now with Superman gone and Steppenwolf/Darkseid's forces just around the corner, the ragtag group of misfits he's collected need a voice of experience to guide them. That's my interpretation at least. I know, it's kinda stupid to ramble on about a throwaway line, but you did ask.
It's a change from the active street vigilante that's portrayed in the comics and the Nolan/Burton films, and unlike his newfound penchant for killing, it's a change that I personally don't mind.
Speaking of killing, I hope he's "matured" past that already. That was what bugged me the most about his character in BvS, as awesome as he was.
I also hope the movie does well enough to not warrant a Sadfleck 2.0. I cri everytiem...