Why do you need to fill the first element of array when implementing heap?

I'm looking at Heap data structure implementation from different sources. What I found is that sometimes it's implemented with the first element of array set to magic (default, unused?) value.

For example here the first element is added in constructor.

And visualization here shows that it's also set to something magic.

However, I've seen many examples without the first element. What's the trick and why sometimes it's there and sometimes it's not?

It is not some magic element but $-\infty$, which is because this is min-heap, in max-heap it would be $\infty$.
• $i > 0 &&$, to lazy eval possibility of the non-existence of two compared elements. With the guardian the check will not be present and the parent will be the guardian. – Evil Jun 28 '16 at 23:39