Computed properties are useful in scenarios where the value of one variable depends on one or more other variables. In this example
We provide the user with an input asking them for a number and automatically return double that number.
Instead of a computed property, we can define the same function as a method instead. For the end result, the two approaches are indeed exactly the same. However, the difference is that computed properties are cached based on their dependencies. A computed property will only re-evaluate when some of its dependencies have changed. This means as long as num has not changed, multiple access to the doubleNum computed property will immediately return the previously computed result without having to run the function again.
You can open the console and play with the example vm yourself.
If we enter in console
app8.num = 299
app8.doubleNumwe will get the console message from the doubleNum computed property first time and also the return value from the function 598.
But after that if we again enter in console app8.doubleNum, we will never get the console message, as we will get the previously cached/computed result always.