Italics are often used for emphasis in writing or — more modern convention — to communicate unspoken, internal monologue/thoughts. Readers either love em, hate em or aren’t fussed. I’m in the former group both as a reader and writer.
But…italics in traditional manuscripts have to be handled carefully. In a regular manuscript you do not use italics. You only use underline. Reason? Copy-editors mark up italics in underline so that the all-important emphasis isn’t overlooked by the compositors. (Nothing says ‘notice me’ like a bloody great line under text.) And, having to underline by hand 300 instances of italics in a book might just add a couple of hours to the copy-editor’s contract and might just be enough to make your book too much hard work. Hey, stranger things have happened!
So…be kind to your neighbourhood copy-editor and get into the habit of underlining all your italics (note: do not also italicise in this case).
Unless your publisher/editor tells you otherwise.
In the case of ARC and synopsis, it is okay to use italics in your hardcopy.