Because Left 4 Dead is a new property for us, we wanted to provide some basic player training prior to the start of the game. Traditional in-game training mechanics didn't make sense for Left 4 Dead, because they would take away from the sense that players had been immediately dropped into a very real, very dire zombie apocalypse. We didn't want a slow ramp-up in gameplay to take away from that tension. As a result, we decided that it would make sense to begin the game experience with a non-interactive introductory movie that could get players revved up and subtly cue them to important gameplay mechanics such as "light disturbs the witch" and "car alarms attract the horde."
Read the full entry[l4d.com]