FORScan lets you retrieve and clear codes when you get a check engine light, but more often people use it to make changes to the software that is running in the various modules in the truck.
Some of the most common modifications made:
Bambi Mode - keeps the fog lights and low beams on whenever you turn on the high beams
Disable double honk - eliminates the double honk when you close the door with the engine running and the key fob not in the truck
Add numerical gauge indication - displays the actual engine temperature above the sweep gauge on the dash
Modify fuel tank capacity - if you replace the factory tank with a larger tank (like the bigger Titan tank), you can reprogram the fuel capacity so that the calculated miles remaining of fuel is accurate
DPF regen - displays percent full DPF so you know when a regen will occur
There are more things you can do, but those are some of the most common. The software is free. All you need to do is buy the correct OBD-II adapter so that you can connect to the tuck.