I imagine that about a week after the first customer was on-boarded into Office 365 they decided that they needed to do a tenant-to-tenant migration. While that is probably not how it went, I suspect I am not too far-off reality with that one.
There are many reasons why an Office 365 customer may need to move some or all users to a new Office 365 tenant. In my experience the most common reason organizations need to do tenant to tenant migrations is because of mergers, acquisitions, or divestitures. These are complicated migrations for a whole bunch of reasons that have nothing to do with the technology of Office 365. I am not a corporate lawyer, so I am not here to address all the legal and political challenges of these migrations. What I can do is talk about the new tools that Microsoft is making available within Office 365 to help with these migrations.
Microsoft first talked about mailbox tenant to tenant migration tools at Ignite 2017. The tool is now available as a public preview. Great progress, but there is a long way to go.