Spectator mode is for spying on your friends.

I spy you my feeder friend.

I love abusing spectator mode to drop in on a friend playing a League of Legends match. It's even more entertaining when they aren't doing particularly well. If you haven't used the spectator system to spy on your friends when you have nothing better to do, you should try it right now. You can even turn on a chat box in the bottom left corner of the UI- making it possible to insult your friends whilst watching them feed.

Note the extra UI addition.

You may observe anyone who is playing and on your friends list. Riot also have a spotlight for dropping into ranked or normal games so you can always find someone to watch even if your friends list is foreveralone. It is possible to fast forward and skip through the boring bits, but you may not progress more than 3 minutes behind the live match. Why? To stop people cheating, of course.

Very useful for spotting new tactics to use.

As many veteran players will probably know, the spectating feature has been 'in production' for a long time by the Riot developers. An immense amount of QQ and demands have been made over the past year by the public on the forums. Since spectator mode is such a basic mechanic to implement, a lot of upset has arisen over the long wait. However, I would say Riot did a good job. It's incredibly easy to monitor both teams, and even easier to follow the flow of the match with the aesthetic scoreboard.