Any time that you want to use the software and you're not the copyright owner. The terms of the GPL grant you permission to use it under some circumstances that

Any time that you want to use the software and you're not the copyright owner. The terms of the GPL grant you permission to use it under some circumstances that you wouldn't have otherwise. It is true that sometimes it is not clear when you are satisfying the conditions in the GPL that allow you to use the software, since there is no case law. But the constraints that forbid you to use the code all come from copyright law, not the GPL itself.