Poker started of as a simple family game of cards but in recent times it has taken the form of a betting game where millions are being put at stake.
Which is why, it becomes necessary for its players to know how to read their opponents expressions that might give them a hit on how and when to deal.
Also, it puts a player at an advantage if he utilizes the 'poker face' where he keeps his facial expressions neutral without giving off how or what he is feeling during the whole period of the game. Even an involuntary display of expression would might give away your position which is why, being careful of them is of utmost importance.