Why do beams supporting structures have "I" shaped cross-sections ?

A beam is a structural element  that is capable of withstanding load  primarily by resisting bending. There is bending force induced into the material of the beam as a result of the external loads, own weight, etc.

 

I-beams are shaped like to letter I to maximize the moment of inertia. This maximizes its resistance to bending and deflection and that is why I-beams are used in building construction as opposed to other beams with the same area. The cross section of structural beams is shaped like the letter "I" to conserve material and reduce weight.

Let us take an example: When such a beam is laid across two columns to support the roof of a building, it has the same weight carrying strength as a beam that has the same height as the "I" but with a widest width possible for "I". Hence, one can build wide open spaces supporting the full strength of the load above.