Why use didSet when you can just mutate a variable and the value changes anyway?

I read through this SO didSet q&a and Apple’s Property Observers and a few other posts. What I can’t seem to wrap my head around is what is the benefit of using didSet when mutating a variable when if you change the variable without using a property observer it’s going to change anyway?

Scenario 1:

var someVal = 0

someVal = 10

// someVal now holds 10

Scenario 2:

var someVal: Int = 0{

    didSet{
    }
}

someVal = 10

// again someVal now holds 10

Scenario 3:

var someVal: Int = 0{

    didSet{

       if someVal > oldValue{

            someVal = newValue
       }
    }
}

someVal = 10

// once again someVal holds a value of 10

The only thing I see in Scenario 3 is that if the condition isn’t met then someVal won’t change. But instead of adding it inside the didSet I can simply do this and the same exact thing will occur.

var someVal = 0

var anotherVal = 10

if someVal < anotherValue{
     someVal = anotherValue
}

// once again someVal holds a value of 10

So other then adding a condition inside a didSet observer what is the benefit?

Well, it's an observer. Many times you want to react to changing a value of a viewController property. Now if you modify that property on 10 different places, you don't have to copy/paste the same code 10 different times - the didSet observer will take care of that.

E.g.:

var someVal: Int = 0 {
    didSet {
        somePostprocessing()
    }
}


someVal = 10

// a bit later

someVal = 15

// etc.

is much better than:

var someVal: Int = 0


someVal = 10
somePostprocessing()

// a bit later

someVal = 15
somePostprocessing()

// etc.

In first case you can be sure that anytime the value changes, the postProcessing will happen. In the second case, even if we would be willing to accept copy/pasting (which is not OK), how can you be sure that you won't forget to add postProcessing in every case you modify the value?