1. No, you don't have to play the card.
Merchant: This card must be played during the counting phase. For the duration of this phase, every commercial area yields five points (instead of three) for players who have the majority in it. In other words, in each of the four areas, the five points may be won by other players than the one who has played the merchant card. The card is discarded at the end of the turn.
2. It does not benefit ONLY others. It benefits ANY player who has the majority (including the player who used the card).
It is worded that way to show that when you can play that card to earn yourself 5 points, other areas not yet counted will score the majority player there (you or an opponent) 5 points also.