This allows developers to easily add custom query data to an page URL without the need to check enabled URL rewriting on their own. Since Twigs `link` filter is just an alias for Pico::getPageUrl(), theme designers can do the same with e.g. `{{ "index"|link("foo=bar&baz=42") }}`.
Theme designers, heads up! Don't forget that the result of the `link` filter is never escaped, so the result could contain unescaped ampersands when passing custom query data. You should pass the result to Twigs `escape` filter when using custom query data.