Alright, it seems some people are confused as to what Johns does in the DCEU or why he’s involved or whatever. The simple answer is that the man is a comic book writer. Not just any writer but a DC writer who knows the characters and their stories. WB hired him for his knowledge of the DC characters, the universe, etc. He even started way back when WB had plans to launch a DC Universe on film with Man of Steel. In June 2008, Warner Bros. took pitches from comic book writers, screenwriters and directors on how to successfully restart the Superman film series. Comic book writers Grant Morrison, Mark Waid, Geoff Johns and Brad Meltzer were among those who pitched their ideas for a reboot. So yes, Johns was already writing up ideas on how to restart the DC film universe starting with Superman. He was even writing episodes for Smallville and Arrow and other DC related shows and also helped out with an episode of JLU.
By the time Batman v Superman was going to be released, Johns was hired as an executive producer for the film. Executive Producer doesn’t mean a whole lot for a film. You don’t have a lot of control in a film if you’re an Executive Producer. An executive producer (EP) enables the making of a commercial entertainment product. The EP may be concerned with management accounting and/or with associated legal issues (like copyrights or royalties). An EP generally contributes to the film’s budget and may or may not work on set. Based off how Johns is, he probably was on set but his contribution to the film was limited. Now, what happened after BvS? He was promoted to co-run the DCEU as well as being a co-producer on Justice League.
Why is this a good thing? Simple. He’s Geoff Johns! He knows his DC stuff. As of mid-2015, Johns and Affleck are co-writing the screenplay for a stand-alone Batman film, directed by and starring Affleck, as well as writing the screenplay for the upcoming Wonder Woman film with Allan Heinberg and writing the story for Aquaman with James Wan. It seems that Johns is mainly writing up scripts and helping future directors with their upcoming DC films by giving them advice. Johns now has more control now as opposed to back then when he was giving his thoughts on how to reboot Superman to contributing to the film’s budget and being on set observing. This is a good thing seeing how Geoff is a great writer and is passionate and cares for the characters just as much as us fans. There’s nothing bad about him joining the DCEU or anything bad about him being there, this just shows that WB/DC cares for their films that they wanted someone who was already on the inside writing comics to be there for the films as well.