Also, larger companies internally operate much like an open source community, having multiple projects that accept contributions from anyone in the company.
Reporting and fixing public issues as part of your employment is a sign that you care about the ecosystem that you are part of, giving back rather than just benefiting from it.
It's not necessarily a showcase of your coding skills, because these contributions are usually small, but it shows that you will get your hands dirty if needed and fix the problem at the root, instead of hacking a workaround in your own code base. It also shows that you have no problem with learning a new code base and delivering code according to that project's quality standards.
If you don't have any public activity it may imply the opposite: that you are just freeloading your community, that you are prone to doing workarounds instead of fixing the problem upstream or that you are unable or reluctant to contribute to other projects.