Programming languages may let you define variables by assigning them. But the scope can differ across languages.

Common Lisp requires one to clearly define the scope. PROGN does not create a scope.

> Why does LET even exist as an alternative to LET*? Why does Lisp even bother making this distinction?

Because there is a scope difference.

One can always do

    (let (a b c)
      (setf a 10)
      ...
      (setf b 20)
      ...
      (setf c (+ a b)
      ...)

Think of

     (let* ((a 10) (b 20) (c (+ a b)))
        ...)

as a short form for

     ((lambda (a b)
        ((lambda (c)
           ...)
         (+ a b)))
      10 20)