The plain keyword is used to avoid having the content being inserted put inside a tag. By default, the content is inserted inside a <span> or a <div> tag (<span> is used if the output does not include <div>, <p> and other such tags.)
Although one could overload the theme_InsertNode_node() theme function, it is at times practical to be able to use both methods.
The plain keyword does not otherwise generate any output.
This keyword primary use is for adding CCK parameters inside HTML code as proposed by one of our users (see here):
The following is the list of currently accepted properties or fields for the Get Property and the Set Property actions. Note that the properties can be specified with either an integer (type 7, requires V5.0+) or a single precision floating point (type 1, V4.0 compatible). And since strings are automatically transformed in a value when required, one can use a string to represent the property number (type 0).
The original document by Steve Hollasch can be found at http://steve.hollasch.net/cgindex/coding/ieeefloat.html
At the very beginning, a company created the SWF format to generate small vector animations on the Internet called Shockwave Flash (hence the name of the format, SWF.) It also included images. This company was bought by Macromedia around 1997 (if I recall properly). This is when Flash v3 was created. Since then, Macromedia created a new version about once a year up to version 8. At that time (in 2005/2006), Macromedia sealed a deal with Adobe which wanted to use the SWF format in their PDF files.