In some cases you may need to get the original MSI file of an app you have installed. A good example of this could be a piece of software you no longer have the install for.
Most times you should be able to locate these msi files in the C:\Windows\Installer folder.
On the left side of this I have ran a powershell line that basically generates the GUIDs of most of the apps installed on the system.
|get-wmiobject Win32_Product||Format-Table IdentifyingNumber, Name, LocalPackage|