What is the difference between fmemopen and open_memstream?

Question

While reading through the GNU documentation on string streams I found two similar functions that do very similar things:

FILE * fmemopen (void *buf, size_t size, const char *opentype)
FILE * open_memstream (char **ptr, size_t *sizeloc)

From reading the documentation, it seems open_memstream should be used for opening an output stream and fmemopen for input. What catches me is the opentype argument that you can pass to fmemopen.

The linux manpage explains:

If buf is specified as NULL, then fmemopen() dynamically allocates a buffer size bytes long. This is useful for an application that wants to write data to a temporary buffer and then read it back again. The buffer is automatically freed when the stream is closed. Note that the caller has no way to obtain a pointer to the temporary buffer allocated by this call (but see open_memstream() below).

So what would be the point of using open_memstream if fmemopen can handle opening an input/output stream?