Yeah, i'm not sure I understand the reason for this as the space seems to separate "function" and "functionName".
Take away the human word functionName what do you have?

function functionName() <- space needed because humans need a space. 
functionfunctionName() <- otherwise, this sucks. 
function() <- not function name, space not needed.

Is there a reasoning for a space requirement in anon functions? Link would be fine.