What is the difference between saying that a duck has webbed feet because it needs them to swim and saying that a duck is able to swim because it has webbed feet?

This is a question of evolution.

The difference is that in the first case, 'has webbed feet because it needs them to swim' speaks to how the web feet may have come to be.

The second case, is ablt to swim because it has webbed feet' speaks to current evolutionary capability.