When should I use deinit?

A deinit() is called immediately before a class instance is deallocated, and it is helpful when you are working with your own resources. For example, if you create a custom class to open a file and write some data to it, you might need close the file before the class instance is deallocated. The most important thing to remember is a class definition can have at most one deinit() per class

from iOS9, removeObserver is called automatically.

It's not required that you implement that method, but you can use it if you need to do some action or cleanup before deallocating the object.

The Apple docs include an example:

struct Bank {
    static var coinsInBank = 10_000
    static func vendCoins(var numberOfCoinsToVend: Int) -> Int {
        numberOfCoinsToVend = min(numberOfCoinsToVend, coinsInBank)
        coinsInBank -= numberOfCoinsToVend
        return numberOfCoinsToVend
    }
    static func receiveCoins(coins: Int) {
        coinsInBank += coins
    }
}

class Player {
    var coinsInPurse: Int
    init(coins: Int) {
        coinsInPurse = Bank.vendCoins(coins)
    }
    func winCoins(coins: Int) {
        coinsInPurse += Bank.vendCoins(coins)
    }
    deinit {
        Bank.receiveCoins(coinsInPurse)
    }
}

So whenever the player is removed from the game, its coins are returned to the bank.