5. They Can Introduce New Characters
As I mentioned at the beginning of this article, it's really weird that Flash will be appearing in Arrow, having his own TV show and his own movie pretty much at the same time. We know that his appearance as a regular on the show will create the road for him to appear in his own spin-off which is most likely to lead to the events of his movie. Arrow made a good job at introducing characters like Deathstroke, The Dark Archer, China White and Death Shot. Considering the way the series is developed, as well as the way it all leads up to the season finale, having some characters from the current movies (as well as introducing new ones in the series) will make perfect sense. This way they can introduce characters that might be difficult to handle in a stand-alone movie (the Martian Manhunter, for example) before they appear in the Justice League movie, maybe even leading up to a proper film.