One has to look back at the history of the project to understand the thinking involved.
Originally, MiaB was created to be a simple, all-in-one solution for running a private email server. At the time, the thinking was ‘why not include the ability to host a static web page as well, since the box serves HTTPS anyways’. If you host your website elsewhere it is as simple as adding an A/AAAA record to point elsewhere.
The project has retained this functionality even though it has grown from what it was initially intended to be.