In Racket and other Schemes parens and square brackets are interchangeable. The Racket style guide suggests to use square brackets in let- or cond-like expression, or generally any expression with a list of lists of fixed length that are not supposed to be evaluated. The expression you posted would become like this:

    (do ([i 1 (+ i 1)])
        ((> i 100))
      (display
        (cond [(= 0 (modulo i 15)) "FizzBuzz"]
              [(= 0 (modulo i 3))  "Fizz"]
              [(= 0 (modulo i 5))  "Buzz"]
              [else                i]))
      (newline))

I personally think this helps a lot with the problem of parsing s-expressions visually.