If government debt grows high enough then an increasingly large percentage of federal spending will be devoted to paying interest on that debt rather than on arguably more useful areas which create future prosperity, such as education and scientific research.
After a certain point this becomes unsustainable and bad things start to happen, ranging from total political & economic collapse (Germany in the 1920s) to hyperinflation (Zimbabwe) to, in the best of the worst case scenarios, a large default/restructuring and subsequent downgrading of a country's debt to junk status (Argentina in the 2000s).
As F.A. Hayek, Milton Friedman and numerous other economists have written, government spending tends to be more inefficient than the free market because the government isn't constrained by profits and losses, which are society's way of indicating whether a company is producing goods and services that are valuable and desired. Of course, there are some goods and services that only the government can provide well, and the current left-right political divide is largely a debate over the point at which government taxation and spending becomes undesirable and inefficient. However, it's undeniable that at a certain level (almost certainly a level below our current level of spending in my opinion) each marginal dollar the government sucks out of the economy is used less efficiently than it would have been used if it remained in private hands.