It's a LISP dialect. Brackets are essential to tell the difference between arguments and function calls... take for example in C: foo(bar(1)) would in lisp be (foo (bar 1)).

If it were to just be "foo bar 1", you couldn't tell the difference between (C style again for clarity): foo(bar(1)) and foo(bar, 1).

Some newer functional languages have a composition operator which allows you to write this without the brackets, but the brackets still are easier to follow in many cases.