As the copyright holder of a piece of code, I can use the GPL to ensure my users won't ever have to wonder about the provenance of my bit of code, no matter where it ends up. When developers adopt the GPL, it should be a conscious decision to protect those rights for future users.

The goal isn't for code to be open source Just Because. Take a moment to consider the motivations behind the FSF's definition of free software, as linked above by drcube. RMS has been talking about this for 30 years.