Allow aliases or additional user accounts to be forwarded
Many people interested in moving to a secure email provider are responsible for managing a family email domain. Sometimes, individual family members (a) do not want to move their address, or (b) are unable to because they couldn't use Tutanota because it is too difficult (compared to simpler, but non-secure products). (I have a family member in this category; they struggle, even after years of tuition, to use Gmail; there is no way I could force on them a different and more complicated email system).
It is always possible to host the custom domain with a registrar or other 2rd party and have some addresses forwarded to one provider, and some to Tutanota. But this removes many of the privacy and security benefits for the Tutanota users.
If it were possible to host a custom domain in Tutanota, but then add pseduo-users who didn't have full accounts, but just had all their email forwarded to Gmail or where ever, that would allow those in the family who want to switch to a secure email provider get all the benefits of doing so.
This has other user cases too: e.g. some people have custom domains which have dedicated email addresses that are monitored by apps, software, device controllers, etc. At the moment, such people cannot use Tutamail.