Parens let you have multiple expressions in one line, which indentation alone could not really express (except perhaps in a Haskell-like with currying, . and $ operators, etc).
For example, I don't see how (map (fn [x] (+ x 2)) (concat (take 5 my-series) etc...)) could be broken down with indentation alone without taking quite a few extra lines.
Comments
Parens let you have multiple expressions in one line, which indentation alone could not really express (except perhaps in a Haskell-like with currying, . and $ operators, etc).
For example, I don't see how (map (fn [x] (+ x 2)) (concat (take 5 my-series) etc...)) could be broken down with indentation alone without taking quite a few extra lines.