IBL gives the possibility to 'fake' shadows and highlights, and the effect works only if you use the appropriate light probe. Typically if you take a photo somewere to use as a background, and when taking the picture you put a probe sphere in the intended position of the figure, the probe will give the light information needed to light the figure correctly for that scene. So, if you take a picture of a large solitary tree with the intention to use it as a background for a scene in which the model leans to the trunk of the tree, you place the sphere where the figure will be. The side where the trunk is will be fairly dark, the opposite side light, the top shaded (by the leaves), and the color tone will match the ambient light.
In the actual application the figure will normally cover the probe.  

There are several issues with this. IBL works best is the shadows are there but not form a definite source, so something flurry from tree leaves in a forest scene or color lights from advertisement boards on a city square.  

The advantage, you need to render only the figure, using ambient occlusion as a tool to generate the local shadows.

IDL requires that you provide actual props to give the shadows and refractions and gives best results in populated scenes with, for example indeed, a sky sphere.