Desktop publishing
In the early days of graphical user interfaces, DTP software was in a class of its own when compared to the fairly spartan word processing applications of the time.  Programs such as WordPerfect and WordStar were still mainly text-based and offered little in the way of page layout, other than perhaps margins and line spacing.  On the other hand, word processing software was necessary for features like indexing and spell checking, features that are common in many applications today.