Printed manuals can be shipped on paper, or they can be shipped electronically as PDFs and then viewed on screen or printed on an office printer.
Printed manuals have a given order of pages. Even if readers don’t read the full document, you can at least expect them to view a number of pages in the given order.
Online help systems are essentially hypertext, regardless of the fact that most online help systems also have a hierarchical table of contents that suggests a particular hierarchy and sequence of topics.
Help topics are typically independent, linked pieces of information. They can be read in any order.