Two important factors to consider when playing a small pair are your stack size, and your opponents' stack sizes.

Small pairs are effective in two ways. With less than 20BB stacks to push all-in pre-flop and looking to take the blinds or possible raise with the blinds without a showdown. Or, if two overcards call you, you will get good pot odds for being a small favorite and adding blinds and antes on top of the value.
Opening with small pairs (22, 33 ,44, 55) from the early position is usually a losing strategy because it's very difficult to play those hands after the flop out of position. Also, when you have less than 30BB and you are facing an open raise it's difficult to find the odds to call and see if you can hit the flop with your hand and go with it.

Small pairs are great hands to play in multiway pots and with deeper stacks. You will flop a set one out of nine times and get good value from your opponents after the flop. Therefore, investing small pre-flop with a small pair can give you great value to play the hand.