For example, watching people play competitive StarCraft is something I've been introduced to a few months ago.
And it's awesome!
It's VERY much like watching sports! It's exactly like watching sports, actually, the only difference is there's no physical effort from the competitors.
But watching Let's Play videos is different, generally. Maybe I want to experience the storyline of an old-school dungeon crawler and don't want to spend tens of hours learning the game and getting through the hard parts. Even better, I can be doing something else (like PLAYING a game) and watch an LP at the same time, I do that often.
Or maybe I simply don't enjoy playing a game but I like it for the storyline or maybe it doesn't run on my PC or maybe I just want to relieve some old memories, or maybe I want to see somebody else experience the game I've just experienced or learn from them or maybe I just enjoy somebody's commentary style.
Splattercat, for example, I watch every single video he releases because I just enjoy his commentary and humor, and his gaming preferences also align very well with mine, so that's a big plus.
Again, not for everyone. But nothing is.