Why does holding ice cream mix overnight prior to churning improve the flavor of ice cream when it is finally made?  This is true, empirically in my experience, even for simple Philadelphia style ice creams with very simple flavorings such as vanilla and coffee. 
Main reason that comes to mind is, for the same reason compound butter's flavour improves over a couple of days: infusion.  Fat can be infused with flavour, and holds flavour incredibly well, but it needs a little time (viscosity of fat vs water).  Otherwise the fat in Serrano Ham would taste just like it would on day one.  By holding your mix/custard overnight you are allowing the coffee or vanilla flavours get infused into the mix.  You can somewhat accelerate this by letting the vanilla sit in hot ice-cream mix for 1hr as suggested by many pastry chefs (e.g. Simple French Desserts, Jill O'Conner).  I suspect the acceleration is due to the lower viscosity of fat at higher temperatures.  The taste does follow the physics here were the higher the viscosity the longer infusion (read equilibrium) takes.  Serrano ham: two years.  Hot deep fryer oil: seconds.  By the way, when it comes to ice-cream, they also recommend aging your mix/custard for up to 48 hours.  It may be worth the experiment to sous-vide your ice-cream batter/mix at say 55C for a couple of hours and compare taste with the over-night version.  Although mouthfeel may play a role in this case, the 'flavour improves over time' effect is present even when crystallization is not happening. 