There are two purposes:

1. Convert all the way to/from wchar_t, to make interfacing with Win32 filesystem APIs easier.
2. Use and handle '\\?\'-style paths. This prefix apparently opts into support for longer filenames. This seems like a good idea for any new programs! (It also opts out of support for using forward slashes as path separators and handling of '..', but kj::Path already handles these, so great.)