Can somebody please explain the "-validate" option to me? How does it work?

[ObiW4nKenobi]

So, I've already downloaded a manifest for a game I own using the DepotDownloader software. And I used the '-manifest-only' command-line switch argument to the manifest file. That has all the SHA1's in it.
So, what is the use case scenario for using the '-validate' switch argument?
And as a followup to these questions. I wanna know if it's possible to use the 'DepotDownloader' software to validate post download.
I have a hunch about what the '-validate' switch argument does. I think it verifies the integrity of all download files in real-time, that is to say, once each single file is finished downloading.
How would I use the persistent manifest to verify all (or even just some files) post download at a later time?
Thanks for reading!