Jeffrey Snover predicted internally in 2014 that PowerShell would eventually be open sourced but it was the advent of .NET Core and getting .NET Core working on multiple Linux distros that kickstarted the process. If you were paying attention it's possible you could have predicted this move yourself. Parts of PowerShell have been showing up as open source:
To be clear, I'm told this is are alpha quality builds as work continues with community support. An official Microsoft-supported "release" will come sometime later.
This is my opinion and somewhat educated speculation, but it seems to me that they want to make it so you can manage anything from anywhere. Maybe you're a Unix person who has some Windows machines (either local or in Azure) that you need to manage. You can use PowerShell from Linux to do that. Maybe you've got some bash scripts at your company AND some PowerShell scripts. Use them both, interchangeably.
If you know PowerShell, you'll be able to use those skills on Linux as well. If you manage a hybrid environment, PowerShell isn't a replacement for bash but rather another tool in your toolkit. There are lots of shells (not just bash, zsh, but also ruby, python, etc) in the *nix world so PowerShell will be in excellent company.
Be sure to check out the coverage from around the web and lots of blog posts from different perspectives!