Alert! First blood. Double kill. Triple Kill. Mega Kill. Overkill. ...Executed. Game over.
They might set the tone of the game, they might always remember to direct you to the objective, heck, they might even feed your ego by lauding every minor triumph you have while playing a multiplayer game, but man, announcers—those disembodied voices you hear while playing something—don't get enough credit.
Can you imagine, say, Team Fortress 2 without it's announcer?
Or Halo without its announcer?
Or Unreal Tournament without its announcer?
What about Mortal Kombat without its announcer?
No! None of these franchises would be the same; announcers are so integral to our experience that not only would it probably feel strange to play these games without the announcers, but I'm guessing that you also read at least some of the intro to this post in an announcer's voice. Unreal Tournament's announcer is so good, just listening to him is as evocative, if not more evocative, than playing most shooters. Now that's an accomplishment.
Okay, yes, the concept of having someone watch your every move and blurt out orders to you is kind of weird. And granted, while announcers can make a huge difference in the way a game feels, not every game has the benefit of having an iconic announcer. Some announcers are bad (or they can get out of hand), and some games make the mistake of trying to copy the way an announcer in another game sounds. Beyond that, some announcers are interchangeable: DOTA 2 does a curious thing where it lets you purchase items that give you a custom announcer. It may be a purely cosmetic change, but still, it's an awesome one. You can, for instance, play the game with Bastion's narrator as an announcer if you'd like.