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Some of it will be about reliability, i.e. the office burns down and Microsoft still hold a copy. Some of it will be about having a third-party that is "trusted" handle the most dangerous part - security. If SharePoint gets compromised there is plausible deniability that "we did everything we should do".

I know for example that some companies will hire subcontractors for high risk parts of a project, just so that there is somebody to blame if anything goes wrong.