Citando Joe Armstrong (grifo meu):
The original reason was that there should be a defined total order over all terms (why? - so that we could write generic sorting algorithms that could order any terms).
The actual order was based on the idea of "complexity" an integer is "simpler" than an atom. a tuple is simpler than a list and so on..
There was no real definition of "simpler" it was more or less the size that an object took in memory (by which measure , should have been smallest, but is not :-).
The actual order is not important - but that a total ordering is well defined is important.