AO stands for Ambient Occlusion, subtile shades that happens when in reality two objects are closer each other and/or are influenced by the light environment.
To fake this in the old programs an AO node was added to show those proximity shadows, with normal lights (infinite or spot) or Image Base Lighting, with the current tecnology isn’t needed anymore but you need Global Illumination.
So, if you want to fake AO, you need the AO mats, if you don’t need or want that or you use GI, use the no AO.