The reason for this, is that it gives to the victim, a choice in the matter. It involves actual violence, or threats of it to the contracting party, or maybe someone close to them. In more recent times pressure of an economic nature has constituted duress. Whatever the label, and whether the principle derives from common law or equity, the effect is the same - the contract is voidable. The action complained of does not have to amount to a wrongdoing or an unlawful action.
Much pressure is known and tolerated and part of everyday life.
But there is a point when the pressure becomes unacceptable, and it could be said that the party is acting unconscionably . It was thought that duress must be to the person and not to goods. Yet money paid to release goods unlawfully detained, has been held to be recoverable. This form of pressure could be just as coercive as any other on the mind of the victim. The idea that there could be no duress to goods is probably now a dead letter.