What is Crossfeed?
With speakers, sound from the left channel will be heard first by the left ear and then by the right ear at a slight time delay and vice versa. The sound that travels to the opposite ear will arrive at the eardrum filtered with mostly low and mid-frequency energy. This effect is called crossfeed.
Why do I need Crossfeed?
Most commercial audio is created, mixed, and mastered in a studio using speakers. Without crossfeed the spatial impressions you get will differ from a professional environment. Because of this you will have trouble referencing professionally made music with headphones.