An act for granting and applying certain stamp duties, and other duties, in the British colonies and plantations in America, towards further defraying the expences of defending, protecting, and securing the same; and for amending such parts of the several acts of parliament relating to the trade and revenues of the said colonies and plantations, as direct the manner of determining and recovering the penalties and forfeitures therein mentioned. (1.1)

The opening line of the Stamp Act lets everyone know what they're there for. This act is going to pay for the costs of "defending, protecting, and securing" the colonies. It's kind of nice this bit of tax code includes its purpose right there. Modern ones don't get that specific.