On second thought, that file is there so you can uninstall SP3 and that is where all the stuff is that it needs to put back to restore you to SP2.
In fact, I recently had to use it. There is a small gotcha that was not discussed before installing SP3.
That concerns IE7. If you installed IE7 before installing SP3, and then install SP3, IE7 is no longer uninstallable on the Add/Remove Programs list.
So, in order to correct a broken IE7, I had to first, uninstall SP3. It restored me to SP2. Then I uninstalled IE7. Then I reinstalled Sp3, and then lastly, IE7. Now, IE7 can be separately uninstalled if necessary.
Bottom line - that big file contains all the backup needed to uninstall SP3.
MS KB