What is the HUD-service responsible for?

The HUD or Head-Up Display was introduced in release 12.04. It is still being developped and perfected and meant to ultimately replace menus in Unity.

This wiki should definitely help you understand some of the intent and portent of HUD (hud-service) as an integral part of the unity architecture. It is meant to be and functions as an "intent-driven interface" meant to facilitate the management of the desktop's applications' menu interface, the GUI end of it being the visible std-out (display).

To most users hud-service remains obscure and a "background thing". In fact its central promise is that it will help tap users' actions' predictability in navigating the GUI.

If you turn that service off you will arguably downgrade the efficiency of your GUI management in terms of responsiveness and RAM usage. Remember that Unity is a rather chunky bit of software and it puts a strain on any system, however big it happens to be.
HTH

You can just remove the hud package: sudo apt-get purge hud. No need to mess with CCSM or the actual hud-service executable (your changes would be overwritten during upgrades).

The HUD is related to the Unity Launcher. For example it analyzes the files you are using and offers them when you press Super.

On my system (Ubuntu 15.10) the hud-service started using 100% of one CPU.

I solved this by installing Compiz Config Settings Manager (ccsm) and disabled all effects and set "Enable Low Graphics Mode" in "Ubuntu Unity Plugin":

sudo apt-get install compizconfig-settings-manager
ccsm