It has always been difficult for the Hero side to win the Final War Arc of My Hero Academia. They not only have to contend with some of the most formidable villains in history, but they also have to rely on intricate techniques in order to gain the upper hand. The results might be disastrous if they lose whatever advantages they now possess.
If Kurogiri is freed, it would be one major problem for the Heroes. Unfortunately, the very worst-case scenario did occur in Chapter 373, "Fireworks," but not quite how anybody had anticipated. It could either be really bad for the Heroes, really good for the Heroes, or a little of both depending on how things play out in the future.
What is Kurogiri's current state capable of?
Kurogiri was a prisoner being kept at Central Hospital. Spinner organized a sizable mob of aggressive Mutant protestors to break into a hospital and liberate the bad guy. Even though his supporters gradually dispersed, he was still able to reach the area where his possible ally was being detained.
Using one of the dismembered hands Tomura Shigaraki wore, Spinner was able to revive Kurogiri. Present Mic, who had known Kurogiri while he was still Oboro Shirakumo, tried reaching out to his old buddy in an attempt to convince him to answer at the same moment. As a result, "Kurogiri" had traits from both his villain and his former hero sides when he first came to consciousness.
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.