Why does holding ice cream mix improve flavor?
Ice Cream Science - Aging the mix was the best reference I could find, though I haven't had a chance to look at Modernist Cuisine and Food and Cooking yet. Basically, it improves mouthfeel, allows more air to be retained (this could be good or bad depending on your opinion of overflow), and helps slow melting. All of the cited reasons have more to do with mouthfeel and texture, rather than taste though.


  (i) Absorption of Emulsifiers
  
  Two important changes take place during the aging process. First, the
  emulsifiers (lecithin from the egg yolks) absorb to the surface of the
  fat droplets, creating a weaker membrane that is more susceptible to
  partial coalescence.
  
  When the mix is frozen in the ice cream machine, it undergoes partial
  coalescence, during which clumps of the fat globules form and build an
  internal fat network (Marshall et. al, 2003). These fat globule clumps
  are responsible for stabilsing the air cells and creating a
  semi-continuous network of fat throughout the product resulting in a
  smooth texture and resistance to meltdown  (Tharp et al, 1998).
  
  (ii) Crystallisation of fat
  
  Second, the fat inside the droplets begins to crystallise. Nearly
  complete crystallisation is needed to promote coalescence of fat
  globules during freezing (Marshall et al., 2003). Cooling mix to 0-2°C
  increase the rate of crystallisation. Barfod et al., 1991, showed that
  crystallisation of fat  in a mix containing 10% fat requires at least
  4 hours.
  
  If you do not sufficiently age your mix, your ice cream can suffer
  from defects similar to those found in mixes with no added
  emulsifiers: less retention of shape and relatively fast meltdown
  (Marshall et al., 2003). It will also be difficult to stabilise air
  bubbles during the whipping stage, resulting in a hard chewy texture.