Standard output is often slightly abused. Its proper use is for programs that produce an output stream, but its most popular use is for debug prints. These really ought to go to standard error (or perhaps to a more specific stream like CL's
*trace-output*). But in most languages that's more trouble, requiring something like
fprintf(stderr, ...) instead of just
printf(...), and it usually makes no practical difference (especially in programs that don't use standard output for anything else), so most programmers don't bother. We'd prefer to do the Right Thing, but not if it's any extra work.
Caml removes this barrier to doing the right thing: its
prerr_* functions are just like
print_*, but print to standard error instead of standard output. This means you can send your debug messages to the right stream as easily as to the wrong one. (In retrospect, this seems obvious — if a language provides convenience functions for one stream, shouldn't it be standard error?)
This particular feature is not important, but the general principle is. Programmers tend to take the path of least effort, even if that leads to bugs (as with C's string functions or unescaped Unix shell arguments) or awkwardness. Any case where they need to be told to do things differently may be a case where the language could prevent a problem by making the right thing easy.