Chrome Store does not accept CRX file but simple ZIP file.
So I am confused why the CRX files exist? Also extracting the ZIP file and loading it unpacked option is still an option.
As far as I know Chrome does not allow extension installation other than Chrome Store.
So what is the purpose of the CRX file?
It's possible to privately host an extension. To do this you need to host the CRX file. There's some information about this in the Google docs for hosting extensions.
If you have a managed Chromebook for example, then you can push out a privately hosted Chrome extension to users. There's an example of this in the Google help docs for managing apps/extensions under the 'Select apps and extensions for your users' section.
By privately hosting the extension it doesn't need to be in the Chrome store.