Quite simply, this is the smartest way to buy from InstallShield.
Purchase orders are a hassle.
New software approval takes time and is barely worth the effort. And then when a new version appears six months later with hot new features you want to utilize, you have to start the process all over again. Then again, maybe you don't. Well, you can. When you upgrade to the latest release of InstallShield Professional - Windows Installer Edition, you can join the InstallShield Subscription Plan.