This option behaves similarly to AlignConsecutiveDeclarations and AlignConsecutiveAssignments, aligning the assignment of C/C++ preprocessor macros on consecutive lines.
I've worked in many projects (embedded, mostly) where header files full of large, well-aligned "#define" blocks are a common pattern. We normally avoid using clang-format on these files, since it ruins any existing alignment in said blocks. This style option will align "simple" PP macros (no parameters) and PP macros with parameter lists on consecutive lines.
Related Bugzilla entry (thanks mcuddie): https://llvm.org/bugs/show_bug.cgi?id=20637