It's a way of cloning an object, so that you get a complete copy that is unique but has the same properties as the cloned object.
var defaultParams = { a : 'b' };
var params = JSON.parse(JSON.stringify(defaultParams));
console.log( params.a ); // b
console.log( defaultParams.a ); // b
console.log( params === defaultParams ); // false
The above outputs false because even though both objects have the a property, with the value b, there are different objects that are independent of each other (they don't refer to the same reference).
The JSON method will only work with basic properties - no functions or methods.