I believe the point is not PLAYING GAMES on the igpu. The real advantage of having one even when you have a discrete card is to be able to USE the PC if your gpu ever stops working, in case you do not own a different pc (maybe to look for help, check mails, browse internet...). If your graphic card ever breaks down, then you fix it or buy a new one in order to play.
But having a discrete graphics card break is not really common, and by the time it stops working you will probably need to change it anyways...