Instancing is simply at tool to reduce size of a draw call, at the cost of always drawing all triangles of all instances, whenever just one is needed.
For example, 1000 triangles, 100 objects, individually drawn, would create 100 draw calls, each passing 1000 triangles to the video card, but these would be drawn when needed and occluded individually.
In instanced setup, this will be drawn in one draw call, passing 1000 triangles only once to the video card, with additional 100 transforms (loc, rot, scale) of other instances. But video card will have to draw all of them each time any of them are visible (or could be visible due to shadows, etc).
Overall data passed from game to video card is roughly 100x smaller, but number of triangles/vertices required to draw would usually increase.
It is good idea to identify if number of draw calls is a problem, before starting to use instancing.