I have a 2004 F150 XLT Supercab with 52,000 miles. I was cleaning it out today and noticed the a ziploc bag that came with my truck. After I started to read it I noticed it was a "Trailer Tow Relay" & it was supposed to be in the fuse box in as Fuse#5.

What is this fuse actually used for & why wasnt it already installed when i bought my truck????

 

If you are towing a travel trailer it will supply 12v at 7-pin connector at rear bumber. This will supply 12v to trailer battery. It will only be active with key on which picks relay.
It's not installed from factory. Could be the extra chance of electric short and extra cost of installing in just some on line...