What is the purpose of a single pound/hash sign (#) on its own line in the C/C++ preprocessor?

Question

I have been looking at the Boost libraries source code, and I have noticed that often there are single pound signs without any preprocessor directives attached to them. I read t

Answer 1

It makes the source code look pretty, that's all.

Highlights the fact that the whole block is a preprocessor section.

And indeed, both the C and C++ preprocessors must ignore # on a line.