It was critical because walt framed Gus for poisoning the boy as a way to get Jesse to think Walt did it, and finally take out Walt for Gus. Walt said Tyrus lifted it from his coat in the lab and Gus poisoned Brock to make it look like Walt did it, to get Jesse to be loyal to Gus, and do away with walt and take over the lab. He pointed out Gus didn’t mind using children to sell drugs, and that he gave the order for Tomas to be killed. This convinced Jesse Gus did the poisoning and he sided with Walt to help get Gus.
Thank you. My short-term memory is not so good anymore. But that helped. ;-)