What does the MSI property/value "REINSTALL=ALL" do, and why do some patches (MSP's) require it?

Question

What does the MSI property/value "REINSTALL=ALL" do, and why do some patches (MSP's) require it?

I understand it from the perspective of its usage with the msiexec /p command, however I don't understand what it is doing under the hood. For example, is it taking all files from the patch and forcing any of its later files (e.g. DLL's) to overwrite what already exists?

Edit: Also, does REINSTALL=ALL look at the Version of the file or the Date Created or Date Modified of the file, to determine which one is newer?

Answer 1

"REINSTALL=ALL" applies to what features are being reinstalled (all features in this case).

By default all files with lesser versions, or not present, that are currently installed will be reinstalled. This can be further specified with the REINSTALLMODE property.

Why it is required for some patches, maybe because they want to force all files to be reinstalled, regardless of whether the version is lower?