Consistency and skim-ability, I think.

die "Unimplemented.";
die "Not implemented.";
croak "Unfinished.";
confess "Still needs work.";
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...then...

perl -nE 'say "$.: $_" if /Unimplemented/i' source.pl
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...vs...

perl -nE 'say "$.: $_" if /\.{3}/' source.pl
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Both will turn up the occasional false positive. But consistent use of '...' will avoid misses when skimming code for gremlins that haven't been caught yet.

It's probably the 2nd least useful operator introduced in modern Perl (~~ is #1), but unlike #1 it does no harm, and at least could contribute to a little less time spent looking for unfinished code.