so basically getter method is same thing as computed property ?

Yes, you are right, as soon as you use a custom getter and/or setter method with a variable, it is a computed property, as opposed to a stored property. From apple dev docs:

Unlike stored named values and stored variable properties, the value of a computed named value or a computed property is not stored in memory

That is, computed properties can not store and return a value associated with itself, but are rather used to manipulate other stored properties indirectly and/or return a custom value.

_{^{This is quite confusing when coming from a language like Objective-C or Java, where you use getters and setters to manipulate a backing variable (in case of Objective-C normally an instance variable with underscore _ prefix), so I hope the above makes things clearer.}}

The question that arises here is: When to use computed properties vs. functions?

Have a look at this stack overflow thread. Here is a simple playground example when I would prefer a computed property over a function, for the sake of simplicity

// Stored properties
var firstName: String = "First"
var lastName: String = "Last"

// Computed property
var fullName: String {
   return "\(firstName) \(lastName)"
}

// Getting the property
fullName

// Alternatively, you could use a function
func fullName() -> String {
   return "\(firstName) \(lastName)"
}

// Getting fullName
fullName()

Properties can also be observed, like explained here.