The Final War Arc of My Hero Academia has always been an uphill battle for the Hero side. Not only are they up against some of the most powerful villains in history, but they've had to rely on complicated strategies to give themselves the advantage. If they lose whatever advantages they have, the consequences could be dire.
One big way things could could go wrong for the Heroes is if Kurogiri is set free. Unfortunately, Chapter 373, "Fireworks," saw that precise worst-case scenario come about, though not quite as anyone expected. Depending on how things unfold going forward, it could be really bad for the Heroes, really good, or a little of both.
What Can Kurogiri Do In His Current State?
Kurogiri was being held as a prisoner at Central Hospital. Spinner led a large mob of violent Mutant protesters in a raid on a hospital to free the villain. His followers eventually faltered, but he was still able to make it to the room where his potential ally was being held.
Spinner successfully reactivated Kurogiri using one of the severed hands Tomura Shigaraki used to wear. At the same time, however, Present Mic, who knew Kurogiri when he was still Oboro Shirakumo, tried calling out to his old friend to get him to respond. As a result, when "Kurogiri" awakened, he took on properties of both his villain side and his former Hero side.
It's hard to say what exactly is happening to Kurogiri. On one hand, he's emitting black mist and claiming to be the protector of Tomura Shigaraki. On the other hand, some of his facial features, and, according to fan translators, his speech patterns, resemble the man he was before he was turned into a Nomu. This suggests that, in a way, both Spinner and Present Mic will be able to reach the person they're hoping for.
If Kurogiri goes free, ha can easily use his Warp Gate Quirk to transport himself and others to the different battlefields. He could easily find the Paranormal Liberation Front's leaders like All For One and Dabi and gather them all in one place. He might even be able to gather some of the other imprisoned villains like Gigantomachia and Shigaraki; at the very least, he could transport villains to their locations who could help break them out. In this way, Kurogiri would completely ruin the Heroes' plan to separate these villains and pick them off one by one. From here, they'd have to do a lot of improvising to find a way to win.
For now, the Kurogiri personality seems to be dominant. However, it's possible that the more Present Mic pushes him, the more of Shirakumo will emerge. If that happens, then the former Hero-in-training could use his new abilities and maybe even his old ones to turn the tide back in the Heroes' favor. Unfortunately, it doesn't seem like Present Mic will be able to bring out his old friend completely until after he's caused all sorts of trouble for the Heroes.