In the main Battle Royale story, there's a group of girls from the Shiroiwa High School class forced to fight to the last student standing whose story is maybe the saddest, but to me personally didn't register much in terms of emotional impact at the time. Perhaps because I didn't invest in any of this group's members, preoccupied as I was by the fates of my favourites.
In this side story, we get to see them all again. But this story is different to the main one. Because it's not as gory and brutal (but don't think you can read this without the main story, this isn't a standalone). On one hand, this might displease those readers that like the extreme violence of the main story, as well as those not interested in slice-of-life glimpses into characters that weren't fan favourites by any means. But, on the other hand, this also proves Takami-sensei can (and does) write stories without extreme gore and violence that can be appealing.
The second major difference is precisely this slice-of-life storytelling. Enforcers is pretty much like an isolated world within the island, a little corner of unearned hope. The seven characters taking refuge in the lighthouse don't have much to do action-wise, so we don't see them doing much regarding the deadly game going round them. We mostly see inwards to their emotions and their memories, particularly those to two pairs of characters. The Yukie/Haruka story is one of (Sapphic) one-sided love, and the Chisato/Shinji is one of friendship. Love and friendship are the theme of this manga, which might be at odds with the reality of having to kill each other, kill your friends, so you can survive. Battle Royale is a world that brings out the selfish worst of people, so this is a welcome contrast.
Personally, it was the backstory of Chisato and Shinji that got me. I said in an update that Enforcers made me sadder than the main story, didn't I? The reason was Shinji.
When reading Battle Royale, of course I had my bets on who was the likeliest to survive. Most of my picks were based on either likability or ability. I bet on Kiriyama based on ability alone (I mean, that psychopathic bastard was incredibly good at the game, belief-defying good at times), and I bet on Hiroki based on likability alone (I mean, not that he was bad at survival, but you could see he wouldn't make it out of the island, he was too good-hearted and good guys die); and some favourites, like Shuya I never bet on (come on, we love this adorable dork, but let's be honest, he'd have croaked the minute he was out in the open if not for Shogo...). But Shogo and Shinji I bet on based on likability AND ability. If someone had to to survive this game, it would be Shogo or Shinji, I thought. If someone were to win between Shogo and Shinji, it would be Shinji, I thought.
My heart got broken and stomped on with mindless brutality when that wasn't the case. Seeing a backstory for Shinji broke it again.
I don't regret reading this, though. It's a good complementary side story, no matter how different it feels. Does it add to the main storyline? Not really, no. We already know what happened to the lighthouse girls, we already know what happened to Yukie and Haruka, we already know how Chisato dies, and we know the fate of Shuya and Shinji. Nothing new is added in terms of plot to the main story. But characters? Yes, this adds to the main story in terms of pure character expansion only, which might not the every manga-fan's cuppa tea. For me, however, it was the perfect type of story to add to the Battle Royale universe.
What is there to add to the main story in terms of plot, really? And I'm talking about the main plotline, not about a story from before or after the island. The island arc itself can only be enriched with character-study type of stories, I think. In that light, this wasn't a bad choice.
Oh, and the art is nice. Much richer than the one for the main story. Maybe it's the improvements over time applied to manga, or maybe it's the artist, but I found the illustration style for this side story much more polished.