1st use: comment
It's said as a response to the gamer's emotional outburst about losing the game (such outbursts include: swearing/yelling and/or throwing the controller/mouse). In any case, the response is unwanted.
2nd use: defense
A person may use the phrase as a defense for causing the gamer to lose, such as by walking in front of the TV or unplugging the gaming device. Being used in this way is like pouring salt on the proverbial wound.
This bane of the gaming world is said by a parent/brother/sister or romantic significant other who does not understand the time and effort spent in the game.
Ryan: What the fuck?! *throws controller*
sister: *shrugs* It's just a game.