What it does is that it distinguishes between those areas that would be totally reflective with the given index of refraction and those that wouldn't, letting you tweak your shader accordingly. I assume you know what index of refraction is, if not you should read up on it on wikipedia.
In most general cases you'd rather want to use the glass BSDF directly, but there are non-standard uses in which the fresnel value can be used for other purposes than to simulate translucent materials of differing density.
I seem to recall someone using it to tweak a car paint shader once, but having read about that months ago I'm afraid I don't have the link at hand. Also months ago, also without a link to it, I recall someone creating an ice shader utilizing fresnel input. Both SHOULD be in some thread over at Blender Artists if you're curious enough to go looking.
Short version: for advanced materials it's sometimes desirable to access the fresnel value outside of the default glass shader.