Since you are posting in the CICS forum, I am assuming you are wanting to know why move LOW-VALUES to the output map before sending it. If this is not what you are asking, please restate your question -- using full, complete sentences -- to explain just what it is you want to know. Making us guess half your question is not a good way to get an answer that makes sense!
From section 188.8.131.52 of the CICS Application Programming Guide
A value is present. The definition of "present" varies slightly with the field component:
* For field attributes bytes, the value must not be null (X'00') or one of the values that can be left over from an input operation (X'80', X'02', or X'82').
* For extended attribute bytes, the value must not be null.
Note: BMS sends only those extended attribute values that the terminal is defined as supporting. Values for other extended attributes are omitted from the final data stream.
* For display data, the first character of the data must not be null.
In other words, you move LOW-VALUES to the output map to ensure there are no values except what you put in.