When we use the LOOP construct with the explicit work area, system need to engage the resources to put the required record in the work area, process it and move it back to the table if the needed. This additional processing time could be saved by using the field-symbol. By using the field-symbols we can save this additional time and improve the performance. Field-symbols are similar to dereferenced pointers in C. While using the field-symbol, system uses the same memory allocated to that particular field in the record instead of moving it to work area and processing.
You can find more about field symbols on:
http://help.sap.com/saphelp_nw04/helpdata/en/fc/eb3860358411d1829f0000e829fbfe/content.htm