Just additional piece of mind, really.

If it's a personal site with no real way for visitors to interact with it beyond reading it's content, I'd say it's not that big of a deal. But if it's a website that allows users to sign in, stores data about them, etc open source is a good idea. As /u/txt said, it's best to know so it can be fixed and that proper procedures can be done.

But if it's just your personal site, where you publish some posts and that's it... I don't think it's a big deal to not publish the code personally because it's just your site and it'll only impact you if something is done wrong.