What is the purpose of the 'mutating' keyword in the context of programming in Swift?

In Swift, the keyword mutating is used in methods of structures and enumerations to indicate that the method modifies (mutates) the properties of the instance.

By default, methods in structures and enumerations cannot modify their properties because they are value types and are passed by value. To allow modification of properties within a method, it must be marked as mutating.

Example:

struct Counter {
    var count = 0
    mutating func increment() {
        count += 1
    }
}

var counter = Counter()
counter.increment()  // count is now 1

Without mutating, the compiler will produce an error if the method tries to modify the properties of the structure or enumeration.