If I am not mistaken, heels were originally introduced to provide a replaceable part on a shoe otherwise made as effectively a single piece: once you wore through the parent material you could not repair it without sewing a patch on it. With a heel you could from then on nail another piece on once the uneven (worn) part had been shaved down - very similar in nature to shoeing a horse (a worn shoe is removed and replaced with a new one nailed onto the hoof). Once heels were introduced, they no doubt became the means for one-upmanship exercises and designer/fashion posing - all a bit like now mobile phones are actually promoted on strength of the variety of colours and ring tones (I ask you !). Hence, people started to make them (the heels) higher, more shaped, wider or whatever.