What is <i> </i> for?

Answer 1

It is used for marking text as different from the surrounding text which can mean a lot of things. In my opinion <i> is not very common and usually used incorrectly (for example for icons which makes no sense).
Here are some examples straight from the people who make up the rules: https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-i-element
<p>Raymond tried to sleep.</p> <p><i>The ship sailed away on Thursday</i>, he dreamt.</p>
<p>There is a certain <i lang="fr">je ne sais quoi</i> in the air.</p>
Note that for emphasis we have <em>:
<p>Last night's dinner was <em>amazing</em>.</p>

Answer 2

It makes the text italic (lean to the right)

Answer 3

Ginfio Technically correct but that's one of those misuses I was talking about.
<address> makes text italic aswell, but you wouldn't use it for that purpose. The same goes for <i>.