Most of the time, the editor would be sufficient but it there's some unsupported style or if you want to add some script code to the page, you'll be able to put your custom css and javascript there.
The landing page editors places any code added in the Custome Head HTML within the head section.
Elements inside < h e a d > can include scripts, instruct the browser where to find style sheets, provide meta information such as viewport meta tag for mobile devices among other uses.
One frequent use is JavaScripts to set defaults for form fields or change page elements.
From the practical standpoint it is like the HTML box in the landing opage editor, just placing the code before the main section.
You should open new threads instead, as it helps later users of the Community. (Threads can only be tagged Correct by the OP.)
The answer is: it depends. Custom Head HTML is not a frequently visited/visible area, so it's easy to lose track of special stuff you put in there. I would concentrate on adding to templates for consistency, to HTML blocks for visibility/manageability, and Custom HEAD only as a last resort.
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