When the limit is reached, it forces Congress to reaffirm its willingness to spend what it is already legally obligated to spend.
In my view (I am not a constitutional lawyer), the president should just ignore the debt ceiling as contradictory. Now, the president is assuming that one horn of the dilemma prevails when there is no logical reason for doing so. A constitutional argument can be made that the debt ceiling is in violation of the Constitution, specifically the Fourteenth Amendment, Section Four, and therefore, it doesn't stand.
The Debt Ceiling Limit is Destructive, Duplicative, and Dumb
Stephanie Kelton | Professor of Public Policy and Economics at Stony Brook University, formerly Democrats' chief economist on the staff of the U.S. Senate Budget Committee, and an economic adviser to the 2016 presidential campaign of Senator Bernie Sanders