Besides wanting to have the odd word italicised at times, there's also things like multilingual scores, where having the ability to put, say, the French in italics and the English in normal text would aid the singer a lot; that's also the standard convention, more or less.
See: Need to put one verse in italics (https://forum.noteworthycomposer.com/?topic=9263.msg66165#msg66165).