By behavior, an organism can scout its options, and explore its space of possible adaptations. Waddington said genetic assimilation, or the Baldwin effect, was about converting acquired traits into inherited traits. What it really comes down to is the natural selection of traits controls. Genetic assimilation bumps up the reach of evolution a notch. Instead of being able to tune the dial to the best trait, somatic and behavioral adaptation gives evolution quicker control over what the dials are and how far and in what direction they turn.
Behavioral adaptation works in other ways, too.
Coyotes creep too far south, or mockingbirds migrate too far north. And then, they stay. Their genes endorse the change by assimilating an adaptation which began, perhaps, as a vague desire. What begins as vague desire can skate dangerously close to the edge of classical Lamarckism when it reaches individual learning. One species of finch learned to pick up a cactus needle to poke for insects.