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 itīs 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. thatīs 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. thatīs 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.
everything starts and ends in the right place at the right time.