Let me rephrase the question: Why would the lack of a clypeus evolve over other members with clypeuses (actually, what is the plural of clypeus?); what would let species without clypeae (maybe the plural? ) survive whilst it's close relatives with clypei (another guess...) died out?Where are you finding/seeing it stated that it's an evolutionary advantage?
Agreed, evolution is series of RANDOM transformations. Unadvantegeous ones usually fail to reproduce and pass in the long term, but neutral and good ones comes to pass(again, usually). There is no need for clypeus difference to be useful, it just happened and those animals with smaller and smaller gap(and in the end without it) just kept reproducing more than the ones with it.Just because an animal developed a trait through evolution doesn't mean it was advantageous. It could be a matter of correlation rather than causation.