Used for: copying the content from one field to another field.
The "copy value" rule copies the value of a certain field to a field of your choice. One example of when this rule can be useful is when a number of items do not have a description, and you want to use the title as the description for those items. Another example is when the field "sale price" is empty for a large portion of your items. With the copy value rule you could easily fill in the "sale price" field with the normal price for those items.