I'll never understand why people would rather watch someone else play a game instead of playing it themselves. Watching a playthorugh of the whole thing takes the same time as playing it yourself.
Like others have said already it's a different experience but the enjoyment of that experience most of the time depends on the personality and/or sense of humor of the guy playing the game. If you don't like the sense of humor or how the guy plays the game for example then there's no reason in wasting your time watching that.
Sometimes though you come across some let's plays that are surprisingly fun to watch from the beginning until the very end, seriously, I was not a big fan of watching some else playing a video game on the Internet too but believe me that there's some guys that can make a game even better with their commentary and seeing the game 100% complete in the process.
And let's be honest, there's series of videos on YouTube that are shit and are ridiculously popular for no real reason and some of this guys that make let's plays videos really put effort into it and they don't even have the recognition for that effort.
Also, I played 200+ hours of P3FES and when P4 came out I was kinda interested in buying it but I was not in the mood to play what basically was the same game as P3 but with a new setting, story, characters and polished mechanics at the time... that's just me though. A shitload of people enjoyed P4 and they couldn't stop saying how fun the game was in forums, it almost seemed that no one really played P3 before (which is not a bad thing).