var sky = summer ? 'blue' : 'grey';
Short, readable. Good.
But what if we got more complex conditions ?
var sky = spring ? 'lightblue' : summer ? 'blue' : fall ? 'lightgrey' : 'grey';
Still short, but it start to be much less readable and difficult to understand at first sight.
So, we can indent our code to keep this readable.
var sky = spring ? 'lightblue' : summer ? 'blue' : fall ? 'lightgrey' : 'grey' ;
Haaaa, much better. We can easily see here our conditions and values while keeping the code quite short.
Has a general rule:
var [variable] = [condition 1] ? [value 1] : [condition 2] ? [value 2] : [fallback value] ;
But, do you really need to do a complex ternary operation ? ;)