"What is the original reason for the construction with the uber-I beams?"
Spanning something where it's going to be either difficult, impossible, or inconvenient to put center piers or another type of bridge.
My Father was a highway engineer for over 20 years and I saw a LOT of those kinds of beams go into place.
That one obviously wasn't secured correctly in either the front or rear cradle, and it torsioned while being moved.