I have made the tutorial on patrols and the head markers for making the NPCS move around and do stuff but what the idle markers get used for and how?

Idle markers can just be placed around for use in Sandbox packages, or they can be used in patrols in place of XMarkerHeadings. The NPC will stand at them, and instead of just standing still and facing a direction, they will play idle's randomly from the list on the marker. This results in your NPC's seeming more active and alive, instead of all standing still like zombies. It's wise to use idle markers in most places for patrols and around dungeons, instead of just XMarkerHeadings.

Well that's much better why is not mentioned in the tutorials?

It is. The Beth tutorial on patrols covers them. I don't think the video does, but the wiki page tutorial does.