It’s not in standard C, so the behavior is undefined.
Some implementation uses it to clear stdin buffer.
From C11 7.21.5.2 The fflush function, fflush works only with output/update stream, not input stream.
If stream points to an output stream or an update stream in which the most recent
operation was not input, the fflush function causes any unwritten data for that stream
to be delivered to the host environment to be written to the file; otherwise, the behavior is
undefined.