Making the title of a Wikipedia article italic isn't trivial and there is a special Wikipedia article about how the make a Wikipedia article's title italic but I wasn't able to find out why some Wikipedia articles' titles are italic. So why are they? What does it indicate?
This template is used to help facilitate the Wikipedia Manual of Style, which lists a lot of different circumstances where Italics need to be used.
Also found on the guidance on Article titles:
Use italics when italics would be used in running text; for example, taxonomic names, the names of ships, the titles of books, films, and other creative works, and foreign phrases are italicized both in ordinary text and in article titles.