Normally, if you perform a system wide shutdown before you install Smedge, then you won't need to restart after running the installer. As long as the program files that are being written to disk are not in use by the machine when you run the installer, there will be no need to restart the machine.
However, if the program files are in use, then the installer has saved the update information with the OS, but the OS can't perform the actual upgrade until you restart it. If you do not restart, then you will not have finished the Smedge 3 installation, and the program may not run correctly. If you browse to the Smedge 3 program folder, you will see duplicates of some files with a prefix like "ci0-". These are the files that are in use, and cannot be updated.
When you restart the machine, the OS will automatically finish the copy process at boot time, and when the machine has finished, everything will be set up correctly and running, with no duplicate files and the latest version properly installed.
If the installer prompts you to restart, and you do not restart this copy process will not be complete, and the program will not be properly installed. Smedge may not operate properly until you restart the machine.