i'm not really sure myself, but i can think of some minor advantages

with `let` you can say

    (let ((x y) (y x)) ...

without worrying about the ordering of the variable bindings. this is more interesting for dynamically-scoped variables; in emacs lisp, for example, you might want to say

    (let ((case-fold-search t) (outer-case-fold-search case-fold-search))
       ...
       (let ((case-fold-search outer-case-fold-search)) (f))
       ...)

so that when you call `f` it doesn't see your case-fold-search binding

with `let*` you implicitly have an execution sequence over the binding forms, but in most cases that's something you're specifying by accident. `let` strongly suggests to the reader that she can consider any one of the binding forms in the list in isolation; she doesn't have to read through the first n-1 bindings to understand the nth one

it's true that, in most cases, all three of them do the same thing, and this is not the most parsimonious approach