It simplifies. 

say "foo: bar" =~ s/^ ( \w+: \s* ) bar /${1}baz/xr;

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vs 
say "foo: bar" =~ s/^ \w+: \s* \K bar /baz/r;

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It helps avoid avoid repetition. 

say "a:b:c:d" =~ s/ :\K [^:]+ (?=:) / uc($&) /xegr;

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vs 
say "a:b:c:d" =~ s/ : [^:]+ (?=:) / ":" . uc($&) /xegr;

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It effectively permits variable-length lookbehinds or makes them faster. 

say "foo: bar" =~ s/ (?<= ^ \w+: \s* ) bar /baz/xr;

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vs 
say "foo: bar" =~ s/^ \w+: \s* \K bar /baz/r;

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The former was an error when \K was introduced. Even now, it's experimental, and there's a maximum to how much it will look behind this way.

In reply to Re: Practical use cases for \K in regex by ikegami
in thread Practical use cases for \K in regex by perlboy_emeritus

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