Software manuals must be free, for the same reasons that software must be free, and because the manuals are in effect part of the software.
The same arguments also make sense for other kinds of works of practical use — that is to say, works that embody useful knowledge, such as educational works and reference works. Wikipedia is the best-known example.
Any kind of work can be free, and the definition of free software has been extended to a definition of free cultural works applicable to any kind of works.