Splitting /usr off is mostly done for access control. Plus if the directories are already split up you have the option to move them around later. /opt is usuall

Splitting /usr off is mostly done for access control. Plus if the directories are already split up you have the option to move them around later. /opt is usually for 3rd party apps that arn't part of your OS distribution. Like apps that come with precompiled binaries and libraries you want to make available system wide for example.