Ambient Occlusion will do a bunch of things for you depending on what you are rendering. It can bring back much needed shading in a physically plausible rendering where the ambient light is overstating its purpose. Ao can also give you nice contact shadows between your objet and object materials.
you how to get more control over your Ambient Occlusion so you can save render times
Setting Ambient Occlusion for the whole scene will affect your render times. The good news is, that you can easily control it in your scene, limiting which objects will receive Ao.
Here, Joren Kandel shows how you can get more control over which objects are using Ao, that can bring your render times back down to a comfortable level.
