Having a custom domain for email also makes it a lot easier to move email provider in the future if you want/need to since your addresses can stay the same as long as the new provider supports catch-all for all the aliases.

This and the online-only are the reasons I no longer use onshape. While I don’t mind for a majority of my stuff to be public it just doesn’t make sense for everything and then you can’t even use it e.g on longer train rides without internet.