To think that only usability determines which media are used is unreal. Cost is important. Online help is used not only because sometimes online help is better than printed documentation, but also because online help is usually cheaper.
The development costs of the source documents are approximately equal for printed documentation and online documentation. The general technical writing tasks are similar (for example, getting information, document design, reworking because of changed software). Inputting the information into a source document is a small part of the documentation process, and it makes little difference whether the final document is printed or online. The page layout for printed documentation adds some time to the project, but it is not large.
What's This? (field-level context sensitive) style help is time-consuming to implement, because each control in the software (such as an entry box, button, check box) must have an ID which must match an ID in the help. If a control is changed, the help must be changed, recompiled, and tested against the updated software. Nowadays, this type of help is not usually developed.
It is possible to use window-level context-sensitive with hyperlinks on a graphic to explain fields—a simulated What's This? help. This is time-consuming to create, and unless the individual popup topics are also listed, the user will not be able to print them.
For the supplier, delivery cost for online documentation is always cheaper than printed documentation. Typically, it is almost free, because the documentation can be included on the CD on which the software is supplied.
However, if the documentation needs to be printed, supplying it on a CD dumps the production cost (time and materials) on users.
To print documentation in batches of less than approximately 1000, digital printing is the best solution.
For large print runs, litho printing is the best solution. Litho printing is good quality, and relatively cheap. The problem is the large setup cost.
The maintenance costs of printed documentation and online documentation such as CHM or WebHelp are approximately the same. For both types of documentation, when the software changes, the documentation must be changed.
However, with What's This? field-level help, updates are more expensive, because the update process is more difficult.