One really good example of a struct is CGPoint; it's a simple value that represents a 2-dimensional point. It has 2 properties, x and y, and can be written as

typedef struct {
    CGFloat x;
    CGFloat y;
} CGPoint;

If you used Objective-C for Mac or iOS app development before, you've almost certainly come across CGPoint; CGPoints hold the position of pretty much everything on screen, from views and controls to objects in a game to changes in a gradient. This means that CGPoints are used a lot. This is even more true with really performance-heavy games; these games tend to have a lot of objects, and all of these objects need positions. These positions are often either CGPoints, or some other type of struct that conveys a point (such as a 3-dimensional point for 3d games).

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Points like CGPoint could easily be represented as objects, like

@interface CGPoint {
    CGFloat x;
    CGFloat y;
}

... //Point-related methods (e.g. add, isEqualToPoint, etc.)

@property(nonatomic, assign)CGFloat x;
@property(nonatomic, assign)CGFloat y;

@end

@implementation CGPoint

@synthesize x, y;

...

@end

However, if CGPoint was used in this way it would take a lot longer to create and manipulate points. In smaller, faster programs this wouldn't really cause a difference, and in those cases it would be OK or maybe even better to use object points. But in large programs where points are be used a lot, using objects as points can really hurt performance, making the program slower, and also waste memory, which could force the program to crash.

The format of the struct statement is this:

struct [structure tag]
{
   member definition;
   member definition;
   ...
   member definition;
} [one or more structure variables]; 

Example: declare the ThreeFloats structure:

   typedef struct {
    float x, y, z;
} ThreeFloats;

@interface MyClass
- (void)setThreeFloats:(ThreeFloats)threeFloats;
- (ThreeFloats)threeFloats;
@end

Sending an instance of MyClass the message valueForKey: with the parameter @"threeFloats" will invoke the MyClass method threeFloats and return the result wrapped in an NSValue.