Racket allows for (), [] or {} in code and it seems that in practice [] is used to distinguish some syntactic forms that have nested parens from normal function calls such as the let and cond forms. So instead of

  (cond ((...) (...))
        ((...) (...)) 
        (else ...)) 

you get

  (cond [(...) (...)]
        [(...) (...)]
        [else ...]) 

which seems to me to be more readable. See here for an example http://docs.racket-lang.org/reference/if.html?q=cond#%28form...