The light bar on the PS4 was originally pitched as a way to communicate things like player health, back when the PS4 was first announced in February. More tangible ideas were conveyed at E3, however, in a demonstration for The Playroom, a series of tech demos included with every PS4 system. In The Playroom, players use the light bar on the DualShock 4 in conjunction with the PS4 Eye to play Air Hockey and interact with little robot men.
We've got some questions into Sony for more clarification on the light's effect on battery life and will update this post when we hear back.
Sony PlayStation 4