Hi Sushma,
In many cases, another system is looking for a specific file name in the folder and might pick an incomplete file before it is completely written by PI.
Temporary file option is a handy option to prevent an incomplete file to be picked up since a temporary file name is assigned dynamically by the system (if not provided explicitly) and when it reaches end of file; then the actual file is created and the temporary file is deleted.
Hope it is clear.
Ambrish