With the CW shows not much, but with the DC movie universe, that would have given some background to Batfleck's Gotham, and some kind of origins story to perhaps some vilains we will see in a future Batfleck solo. This way we jump right on the middle of his career, no extended introductions to the other characters in his movie, if you want to know more about them, you can simply watch the show.
Same could work with Arrow, if they could connect it with the JL movie, they can have him do his thing in the movie, and if people like him, they can go watch the show, if they don't, that's the end of it and WB doesn't waste money on an Arrow movie either.