Because the field is cleared automatically and it is mandatory, it forces the user to provide a response. The “Action / Response” field on the issue entity is a typical scenario where you want to force this behaviour since you want the user to provide details of why they are updating the issue, i.e. what they have done to move the issue forward.
You can then review the history of this field for the issue to see all the work that has been done on a particular issue throughout its life.
Certain reports, for example the “Issue History Report” is also specifically geared towards this and will show you a history of this field over the course of its life.
The automatic clearing of the contents of a Free Text field can be turned on or off for any Free Text field by the System Administrator, on your instance. Access the following knowledge base article for details.