This is certainly an infrequently encountered macro, but the example and description in the CLHS are quite helpful.  The macro is used to defer warnings until multiple compilation units can be processed, to avoid spurious warnings, for instance when there two compilation units call each other’s functions.

If you, like me, try hard to eliminate all warnings during compilation, this macro can be useful.  Warnings during compilation can often indicate real problems, but if your code in its normal state is generating pages and pages of them, a new warning might be easy to miss, so anything that eliminates spurious warning messages is helpful.