For me, I had to use it to figure out how to reassemble my Mossberg 590A1 shotgun after disassembling it for cleaning. I swear, that thing is like a crazy puzzle if you don't know what the heck to do with all those parts. But the video I found was exceptionally good, easy to follow, and it did what two gunsmiths at the local range couldn't figure out until they looked on YouTube themselves (Link in case you're curious). I was actually wary of using that shotgun for range practice because of the assembly difficulties we had before, but now it's no problem.

And yes, like the guy said, the gun manual for the Mossberg 590A1 is crappy and "isn't worth the paper it's printed on."