Because the tree structure of the parenthesized expression matches the structure of the query, manipulation of this tree using editor support such as Paredit is useful.
Lispers don't always realize that too many parens/brackets is a code smell in almost any other language. It makes people unnecessarily wary of (stuff (like 'this)).
I dunno. If I have a "complex" condition with more than two parts to it, I prefer to be liberal with parenthesis, rather than expect any future maintainer to remember all the operator precedence rules.
