Let's say you have a class you are developing and want to reuse it in multiple targets, particularly if you work with RT targets. You want to do most of your troubleshooting and development on the PC (assuming that the class has nothing hardware-specific) and when it works well you plan to do some final testing in the actual RT target.
So, you create your test project, with the PC target (with the class and the tester VI). Now you add the RT target and reuse all the VIs and the class. BAM! LabVIEW locks the class and you can't make any more changes until you delete one of the targets of your project. You can't even create two projects (one for the PC another for the RT), because the class will be locked.