Think of it literally like live action comic books. Sure, characters pop into other characters' titles all the time, there are events and cross overs, but for the most part each title is it's own story within the same universe. It doesn't have to drive forward any plot.

Having said that, I don't think Suicide Squad was useless in terms of moving the DCEU. For starters, it intrdouces magic. So now when Billy Batson meets a wizard and gets the ability to turn into a muscle-bound superhero with the shout of the word "Shazam!" audiences are now more likely to accept it. Same with Constantine, Zatanna, Etrigan. They're not looking for way to explain it with science and come up with a convoluted story. They just said "Magic exists here. Deal with it." Enchantress is a being thousands of years old and casting spells and Katana's sword literally traps the souls of those that it kills.

Second, We're introduced to a bunch of villains in the this particular universe. Now if they introduce a new villain in a Batman movie and they think it'd serve the story well to have Deadshot show up, you don't have to worry so much about developing Deadshot as much. We've already been introduced, we know what he's all about, so now it's less likely the movie suffers from "Spider-Man 3 syndrome".

Third, world building. Not only are we now introduced to magic and new usable characters, but we got looks at places like Midway City, Arkham Asylum, Belle Reve. It has literally scoped out the map beyond Metropolis and Gotham.