[Question] Italics on auxiliary verbs

As far as I can tell, italics are used as an alternative to quotes or double quotes. However, there is one usage that perplexes me: when they're used on auxiliary verbs. For example,

• I didn’t even know she had a boy friend.
• Oh my god, they really can write!
• If you can do that, you should.
• But what I can guarantee is that...

I've seen this kind of writing on Reddit and Lemmy for quite a while, but still can't grasp it: when and how?