Suppose you have a text, for example an introduction, with some word (i.e. titles) in Italic in it. Now you decide that this text must be in Italic. So, all the titles in it that were in Italic must be in plain.
In CSS this is easy to do; you'll have something like span.title { font-style: italic; } with the variant p.intro span.title { font-style: normal; }.
I think it's a necessity quite frequent. In other similar scenario you may have to use different colors, different weights.