Why do we pass reversal enteries?

At times some incorrect documents might have been entered in the systems.

If you have entered an incorrect document, you can reverse it.  Note that R/3 can reverse a document only if the following conditions are met:
   
- Contains no cleared items
- Contains only vendor, customer, or G/L line items
- Was posted within the FI system
- Contains only valid values, such as business areas, cost centers, and tax codes
   
Ordinarily, you post a reversing document in the same period you posted the original document.  The period of the original document must be open to post a reversing document. If the period is not open, you can overwrite the posting date field with a date in an open period, such as the current period.
   
Reversal can be done individually - FB08 or Mass F.80.
   
If the document to be reveresed contain cleared items, then cleared item must be reset before the reversal of document.