Please can any one tell me what is conversion programs and where it is useful.

Conversion takes place when converting the contents of a screen field from display format to SAP-internal format and vice versa and when outputting with the ABAP statement WRITE, depending on the data type of the field.

If standard conversion is not suitable, it can be overridden by specifying a conversion routine in the underlying domain.

A conversion routine is identified by its five-place name and is stored as a group of two function modules. 
The function modules have a fixed naming convention.
The following function modules are assigned to conversion routine xxxxx:

CONVERSION_EXIT_xxxxx_INPUT
CONVERSION_EXIT_xxxxx_OUTPUT

The INPUT module performs the conversion from display format to internal format. The OUTPUT module performs
the conversion from internal format to display format. 

If a screen field refers to a domain with a conversion routine, this conversion routine is executed automatically each time an entry is made in this screen field or when values are displayed with this screen field.

eg . refer to the vbak table. 
Go to SE11 -> VBAK -> Display mode -> double click on the data element VBELN_VA 

Double click on the domain VBELN

You can find the conversion routine in the Output characteristics. 

Related ABAP Topics:
Deleting Abap Report Protected Variant

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