to try an answer of your question:
Simple : What is the function of HDRI basicly
hdri means you use a texture and wrap it around the whole scene (environmentBall). your texture is now used to enlighten your object from all sides with different colors.
so unlike with a point/spot or direct light which represent just one finite point as origine in space or a directional lightVector the HDR Image simulates real world light coming from all directions. simple enough because it is wrapped around your scene and it´s pixels are used to generate different lightcolor.
you can recreate a very rudimentary hdri light setup just with spotlights. place them around in form of a light dome and give every spotlight a slightly different color. this but is much more time consuming to do and far away from being realistic. that´s why hdr images are photographs taken through a fishEye lense or of a mirror ball to capture a whole 360° environment. that from you get a texture which you use in maya on an environmentBall wrapped around your scene. that´s why such renderings look so realistic (often), cause you simulate real world light situation. to take full advantage - use Final Gathering, which is a technique to simulate bouncing light.
have fun!
_D
everything starts and ends in the right place at the right time.