LTT is one of the biggest YouTube channels and commands a lot of influence in the PC gaming community. The parent organisation actually has a lot of channels on youtube and they pump out a ton of videos under a few different names so even if you don't recognise the LTT name you might have seen some of their other videos just in passing.
Why is it a big deal? Because the accusations are big, given their influence on peoples' purchasing decisions.
I really do wish allegations of this level of seriousness could go through the legal system before being presented to the court of public opinion. If he's actually assaulting 16 year olds then he needs sending down!