According to gcc's manpage i686 encompasses everything from pentium pro onwards, so this command would only be relevant if you were running on a non-pro pentium (i586), a 486 (i486), or a 386 (i386).
So, as the OP indicates, this command isn't appropriate for everyone, but is surely appropriate for a few people.
Additionally, this would work fine on BSDs - we aren't so alien as people make us out to be =0
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