Some hobby server admins like the idea to self-host their emails at home. Maybe because they don’t trust common e-mail hosts or they want to have full control over their hardware. Although this sounds reasonable at first, I do not recommend running a mail server at home.
In most cases running an own CA (certification authority) is not advisable. But there are exceptions: If you want to secure internal services of your company, using your own CA might be necessary. During my employment at ADITO Software GmbH I created a tool for X.509 certificate management. The root certificate of my tool had to be imported into every PC of the company. Unfortunately there are some pitfalls which I did not expect, but after some research I figured out how to import the new CA to Linux- and Windows PCs and to every major webbrowser.