Reasons I watch wow videos:
To find out how to find some item or complete a quest I'm stuck on or just do it faster (ie guides).
To find out more about interesting lore I encounter in-game. Nobbel is really good at this, Blizzard should pay him.
To be entertained, usually by Machinima.
To listen to the music.
To learn raid tactics.
To learn how to farm gold, mounts or pets faster (take gold farming videos with a pinch of salt).
To learn some clever trick which will allow you to get added value out of the game (eg exploration videos).
Following on from this, if I get entertained by a youtuber I will start to just like them and watch them do almost anything.
It is fun watching Nixxiom/Moocluck or Nobbel/Fury level because the fun they have in-game is infectious. However, I need to know the people involved before I watch relatively uninteresting gameplay.