Although you can often call profitably with a small pair against a long line of players in hold 'em, when you're in late position and no one has entered the pot, it's different. Then, you should usually raise, not just call.
The reason is that against many players, you're trying to take advantage of pot odds by calling and seeing the flop. You realize that you'll almost certainly need to improve your hand to win against that many opponents.
But when you're in late position, you can raise hoping to end up one-on-one or to win the blinds outright. If you do end up against just one opponent, there's a good chance your small pair might be enough win the pot, affording you an extra chance to win that you would seldom enjoy against many opponents.
The raise is designed to chase players out and give yourself that extra chance to win.
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