Normally you may have more than one camera deployed in different places in the field.
After you exchanged the SD cards and later on checked them on your computer you may have forgotten where those pictures were taken.
The Serial No. parameter setting takes care of this issue. You can use the combination of 4 digits and or letters to code the location and record it in the photos (e.g. YSP1 stands for Yellow Stone Park site one). This will help you to identify the location when reviewing the photos off-site.
