@Ostrich has a great observation:

It's kind of hard to understand the purpose of protocols when you first learn about them,
and just create one for no reason other than that you were told to.

So true!

Here's an example from my Playgrounds to help me process Protocols.

This is a simple fruit structure:

struct Fruit {
    var name:   String  // Watermelon
    var icon:   String  // 🍉
    var color:  String  // Tricolor 
}

// Conformance Grouping. Fruit behaves like a CustomStringConvertible
extension Fruit: CustomStringConvertible {
var description: String {
        "This \(icon) " + name.lowercased() + " is " + color + "."
    }
}

Simple enough. Now create a simple fruit bowl.

let banana     = Fruit(name: "Banana",       icon: "🍌", color: "Yellow"  )
let apple      = Fruit(name: "Granny Smith", icon: "🍏", color: "Green"   )
let peach      = Fruit(name: "Peach",        icon: "🍑", color: "Pinkish" )
let lemon      = Fruit(name: "Lemon",        icon: "🍋", color: "Yellow"  )
let watermelon = Fruit(name: "Watermelon",   icon: "🍉", color: "Tricolor")

// Check your objects
banana.description  // This 🍌 banana is Yellow.
apple.description   // This 🍏 granny smith is Green.
// Because Fruit behaves like a CustomStringConvertible
// It knows how to describe itself.  No need to add .description
peach   // This 🍑 peach is Pinkish.
lemon   // This 🍋 lemon is Yellow. 

var fruitBowl = [peach, apple, peach, watermelon, lemon, banana, banana] // some random order

Run this in Playgrounds and you'll see you have an array of Fruit objects. So far, so good.

Now try some common array functions on your fruit bowl.

// Common array functions
fruitBowl.randomElement()!.description  // Fruit struct works great in an array.
fruitBowl.first!.icon                   // Fruit struct works with array.

// This line tosses an error!
// Referencing instance method 'sorted()' on 'Sequence' requires that 'Fruit' conform to 'Comparable'
fruitBowl.sorted()

How do you sort fruit?

It's easy to sort Strings. Simple to sort Integers. But Swift has no idea how to sort Fruit. By color? By icon? By caloric density? You may be tempted to write your own Bubble sort routine to compare two Fruit objects and generate a new sorted array. This is a LOT of work. Plus, in future applications, you'll have to write your own sort routines to sort Books, Recipes, BucketList items, etc.

This is the strength of Protocols. Swift boffins devised a protocol named Comparable. In short, the Comparable protocol is a collection of rules that allows Swift to compare two of the same objects. By making your Fruit structure conform to the Comparable protocol, you'll gain all the benefits that Swift provides for sorting objects! Free!

In other words, make your Fruit object Behave Like something that can be compared.

Let's conform!

So add this to your Fruit playground:

// Keep your original structure.       
// But tell Swift that your struct now conforms to the Comparable protocol.
// To Behave Like something that can be compared, Swift requires you to add one function.         
// This function takes any two Fruit objects (left hand side and right hand side)        
// and you tell Swift which one is higher in the sorted order. Swift handles the rest.
extension Fruit: Comparable { // <-- Also Conformance Grouping
    // This function is required to conform to the Comparable protocol.
    static func < (lhs: Fruit, rhs: Fruit) -> Bool {
        lhs.name < rhs.name // <- This is how you compare two fruits. Must return a boolean
    }
}

Now, when you run the line below, Swift knows how to sort your Fruits. All you had to do was add a Protocol to your existing Fruit object. In otherwords, a Fruit structure behaves like something can can be compared.

// Because Fruit objects conform to comparable,     
// you can, well, compare them and sort them. Easy!
fruitBowl.sorted().forEach { fruit in
    print( fruit.description )
}

Sorted Results

When I run the line in Playgrounds I see:

This 🍌 banana is Yellow.
This 🍌 banana is Yellow.
This 🍏 granny smith is Green.
This 🍋 lemon is Yellow.
This 🍑 peach is Pinkish.
This 🍑 peach is Pinkish.
This 🍉 watermelon is Tricolor.

More Behaviours

If you want to show your fruits in a List view, Swift may want to know how to identify one piece of fruit from another. To do this, you'll have to make your Fruit conform to the Identifiable protocol. Your Fruit object needs to behave like something that can be identified, or distinguished from another, similar object.

This doesn't require a function. Identifiable only requires your struct to have a property named id that you guarantee is unique.

Keep Coding!