Since object-oriented languages have the concept of an inheritance, in any inheritance hierarchy there is a root class. In Java, the default parent class (if none is provided) is java.lang.Object
, whereas in Objective-C, if you don't explicitly declare a parent class, you don't get one. Essentially, your class becomes a root class itself. This is a common mistake among Objective-C newcomers, since you normally want to inherit from NSObject in such cases.
While often problematic and puzzling, this actually allows quite a bit of flexibility, since you can define your own class hierarchies that act completely differently from NSObject. (Java doesn't allow you to do this at all.) On the other hand, unless you know what you're doing, it's easy to get yourself into trouble this way. Fortunately, the compiler will provide warnings if you call a method not defined by a class with no declared parent class, such as those you would normally expect to inherit from NSObject.
As for the "use" of NSObject, check out the documentation of the NSObject class and NSObject protocol. They define common methods used for object allocation, memory management, comparison, hashing, printing descriptions, checking class membership, querying whether objects respond to a selector, etc. Basically, NSObject is "good for" providing the core functionality of Objective-C objects free of charge.