A server that shows the source of an error shows the source of an error to whoever asks. The switch is not a convenience, it is a permission with a boundary.
About 1 min of reading
Prove the debugger is refused
Write the mode
Before you run it, say out loud what it will print. Then open the answer.
Attempt to reach the debugger
Before you run it, say out loud what it will print. Then open the answer.
Write the test as a case
Before you run it, say out loud what it will print. Then open the answer.
A switch set per request is a switch that can be set by the request. Creation is the last place the input cannot reach.
A test that asks for the debugger and expects a refusal. The test is the proof, and the proof is in the suite.
The debugger executes code on request. Executing code on request is the most dangerous thing a server can do.
The error is a stack trace in a log. The cost is reading a log instead of a page, and the reading is minutes.
It is allowed on one host and refused on the rest. The host is the boundary, and the boundary is a number.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.