While someone else can likely explain it better than me, I'll take a shot at it.
Lets look at Brandon here since he is who I was talking about. Brandon makes content that appeals to the 15-25 males; most of his content is humorous and filled with stuff that interests young men, but he often times gets into discussion regarding gun laws and regulations. This is a very important thing to teach young men who have an interest in firearms. Getting it into their heads that gun rights are constantly under attack and all regulations are based on some form of purposeful ignorance or malice is good counter-signaling to the constant MSM points about how dangerous firearms are.
We also have someone like Demolition Ranch; Matt makes content that appeals to much younger males, I'd say around 10-14 if not younger, and his content is mostly devoid of politics or discussion of gun laws, excluding small comments, and his content at large is a lot more goofy and wacky, but he still shows aspiring gun owners or even kids with a passing interest that guns can be fun when you are safe.
Essentially, Youtube gun content is propaganda. I know I am starting to sound like a Twitter user here, but stick with me. All MSM and the vast majority of online content regarding guns either glosses over private firearm ownership or demonizes it; the media circus regarding mass shootings beats it into people's heads that guns are scary and black rifles should be banned. Guntube gives a very important opposing view and force to that narrative, pushing that guns are cool and fun as long as you are safe, while also speaking out against laws and regulations that are anti-gun. This content does not usually appeal to longtime gun owners; those types will watch people like Ian or Paul, but it appeals to young aspiring gun owners, the next generation who will have to deal with retarded pushes for gun control.
I hope that was coherent.