Dunno if you want a serious answer, but 'press start' titlescreens that start up an animation if you leave it unpressed too long are a throwback to when if a screen showed the same image for too long, it would fry the image on to the screen and leave a little ghost image, so screensavers were a screen saver. This allowed one to demo software and just leave it running without worrying about damaging the product hardware.
These days however it is totally unnecessary.
So that if you leave the room to make yourself a tea or something while the game is loading, your won't miss the cut scene / beginning of the action / lose the game because it started without you present.