There are a lot of advantages to using the FASTOEM property. It's not just about the time, but it also offers the following features:
Skip System Restore as it is not needed for the first installs.
The source files are deleted after the installation.
You don't get to see any UI during the installation
The installation is performed in the per-machine installation context.
Used for first-time installs
You might also want to take a look at the official documentation of the FASTOEM property to learn more about it.