Learning (development) allows an organism to build a more complex
phenotype than
it could otherwise, given a genotype of a certain size;
environmental regularities can do much of the work of wiring up
adaptive behavior-generators.
Learning allows an organism to make use of the past as well as the
here-and-now through the creation of
episodic memories which can be retrieved later on.
Learning allows an organism to adjust its behavior faster than
natural selection would allow to changes in (a) the environment
and (b) the organism's body.