Although it is true that the sentence was used to test the dot matrix printer (which some companies still use), the reason for =rand (200,99) changing into several hundred pages thereof is not for testing purposes.
You have found a virtual Easter Egg.
Easter eggs are messages, graphics, sound effects, or an unusual change in program behavior, that mainly occur in a software program in response to some undocumented set of commands, mouse clicks, keystrokes or other stimuli intended as a joke or to display program credits. They are often located in the "About" box of a software. An early use of the term Easter egg was to describe a message hidden in the object code of a program as a joke, intended to be found by persons disassembling or browsing the code.
For example, there is a flight simulator game within Microsoft excel 97. Do a search for virtual Easter Eggs and you are going to find a lot of interesting things similar to this one.
Here's a site that lists a few from different programs and OS versions.
The link below is more interesting, so rather check this one out: