No purchase necessary. A purchase will not increase your chances of winning.
The Language
You have read it a thousand times without reading it once. It rides along the bottom of soda cups and cereal boxes, buried in the official rules of every giveaway a brand has ever run, spoken in the auctioneer's blur at the end of a radio ad.
It looks like fairness. It is actually a confession and a defense.
Plain-English Translation
The company running this promotion would very much like you to buy something. It has arranged a prize, and a random drawing, and a great deal of packaging designed to make you feel that spending money is part of how you win.
The phrase exists to say, under law: it isn't. You can enter without paying. And because you can, this promotion is not the thing it otherwise would be, an illegal lottery.
That's the whole trick. "No purchase necessary" is not a courtesy. It is the single load-bearing sentence that keeps a marketing giveaway on the right side of a criminal statute.
Before This Language Existed
Americans used to love a lottery. In the colonial and early republican eras, lotteries were ordinary public finance; they helped fund Harvard, Yale, roads, bridges, and pieces of the Revolutionary War. Buying a ticket was closer to civic participation than vice.
Then came the backlash. A wave of nineteenth-century moral reform, capped by the spectacular corruption of the Louisiana State Lottery, turned public opinion hard against the whole enterprise. Congress responded in the 1890s by making lottery materials nonmailable - you could not use the U.S. mail to run one - and by the turn of the century, state lotteries had been legislated out of existence nationwide. (No state would operate a legal lottery again until New Hampshire revived the idea in 1964.)
That left a category, defined by courts across the country with unusual consistency, that businesses were forbidden to touch. A lottery has three elements:
This is the constraint every promotions department in America has worked inside for over a century. You want the marketing power of win a prize: the prize and the chance are the entire appeal. So the one element you can afford to surrender is consideration. Take away the requirement to pay, and your lottery legally becomes a sweepstakes.
The incantation that performs that removal, in writing, is *no purchase necessary*.