What is the purpose of using extensions in programming or system development?

Extensions in programming, especially in iOS, are used to add new functionality to existing classes, structures, or protocols without the need to modify the original source code of these types. This helps improve code readability and reusability, as well as organize functionality logically.

For example, you can add methods or computed properties to standard types like String or UIView to extend their capabilities:

extension String {
    func reversedString() -> String {
        return String(self.reversed())
    }
}

let greeting = "Hello"
print(greeting.reversedString()) // Output: "olleH"

Thus, extensions help structure code, add new features, and support the principles of openness/closedness (Open/Closed Principle).