A number of commenters (at least one here and several elsewhere) have asked why we need a debt ceiling. If the Congress wants to spend less, why don’t they just go ahead and spend less?
The answer is that different spending programs command different majorities. Snip and Snap vote to fund rabbit hospitals; Snap and Snurr vote to fund trapeze subsidies; Snurr and Snip vote to fund lava lamp research. Plausibly, they’d all prefer to eliminate all these programs. Even if Snap thinks rabbit hospitals and trapeze subsidies are both great bargains, he might not be so happy about getting two for the price of three.
Individual shoppers (at least the perfectly rational individual shoppers we meet in our economics textbooks) have no reason to constrain their own behavior. If you buy the Oreos, it’s because you wanted the Oreos, so why try to stop yourself? But legislators — even perfectly rational ones — are not like individual shoppers. They shop with other people’s money and — more importantly for this issue — other people shop with theirs.
So it can make perfect sense for legislators to constrain themselves by agreeing in advance that they won’t spend more than $X in a given year. A debt ceiling is not exactly the same thing as a spending ceiling, but in practice it can be pretty close.