It's not for the hell of it.

[ ] does not mean method call, it is the syntax for a message send.

Objective-C is a super set of C, adding an Smalltalk like object system to C. The delimiters say "I am sending a message", which is different to a method call. Also, without them the language would be much more difficult to parse, and future changes to C could break the language. It's lasted well (first appeared in 1993). Not as long as Lisp, perhaps it needs more [ ] :)