The primary goal of a Fluent Interface is increased readability.
When used for constructing objects, the choices available to the caller can be made clearly and enforced via compile-time checks. For example, consider the following tree of options representing steps along the path to construct some complex object:
A -> B -> C -> D -> Done -> E -> Done -> F -> Done. -> G -> H -> I -> Done.
A builder using a fluent interface would allow the caller to easily see what options are available at each step. For example, A -> B is possible, but A -> C is not and would result in a compile-time error.