The ability to have a friend (or coach) spectate your games in League of Legends feels like it should be powerful. It feels like you should be able to get some sort of material advantage out of that extra information. If spectating was live it would be easy to get such an advantage... You could call out ward locations in order to allow gankers to avoid the wards, or to allow pink wards to be placed to kill them off. You could see incoming ganks coming and get to safety in time. You could know when the enemies were in a bush and keep the players from dying in a 'facechecking the bush' situation.
Riot delayed spectating mode by 3 minutes which, thankfully, neuters those options. Wards only last 3 minutes so there will never be a point in time where the spectator can see a ward and the ward will still exist in game. 3 minutes is a long time for specific player movement to remain relevant... If a gank is coming it'll have happened and be done long before the spectator even sees it start to develop.
It still feels like something has to be there. We had 7 people online last night so I played one game with people watching and then watched another game while in a Skype call with the players. I was trying to think of some way to help out and wasn't really finding anything. Some things do take more than 3 minutes to develop (blue/red buff respawn in 5 minutes, dragon in 6 minutes, and Baron in 7 minutes. Theoretically the spectator could time out those deaths precisely and let the team know precisely when they'll respawn. This certainly could help, but for the most part it's information the players should have anyway. Having ward coverage on the monsters that matter so you can time out their respawns is important and something that can be done in game. (It's not something I do myself, yet, but I _could_ if I was better.)
One thing you can see easily in spectator mode but not in game is gold earned numbers. But does that really matter? Can we turn knowing the gold differential into an in game advantage? I guess if you're down a lot you could play less aggressively? But again, that's something you should be able to tell in game...
Maybe we can't get an in game advantage, but can we learn? I could imagine getting a stronger player to spectate a game and then point out things that could be done differently. Even 3 minutes delayed it could be enough to try some new things out? I'd like someone who knows how to play against Riven to watch a game, for example, since I seem to just lose whenever I'm up against Riven.