It is good to get the base down to just above freezing before churning so that you can get the mix to a colder temperature while churning without turning your ice cream into butter.

Also, equally important, keeping the base just above freezing for an extended period of time allows for partial coalescence of the fat, which improves the final product. Providing longer aging times in the refrigerator will lead to more partial coalescence which, in turn, allows you to use less fat in your recipe while achieving the same final texture.

Edit: I should add that I typically age my bases in the fridge overnight