> Why is it so important to make it immutable?

Because it never changes. It's not strictly speaking necessary, but code is often cleaner when a variable is directly initialized and then never mutated (which is how it works in the functional world), and if a variable is const (or even better, immutable), then it's protected from accidental mutation and will potentially be better optimized. There are plenty of cases where that's not appropriate, but with a variable like this that's clearly meant to be initialized to something and then used without being mutated, it's just better to make it const or immutable if possible.

> As a side comment, I find no fault with multiple ternary operators.

I do nest them sometimes but tend to avoid them due to how ugly they start looking, and nested ternary operators start to get more error-prone - though I'm quite comfortable with the precedence of the ternary operator and am generally a bit surprised on how many people find it's precedence to be unclear (e.g. Andrej's example is quite clear to me, and I never would have used parens there).