What is a DPF/DEF/EGR and why would I delete it?Updated a year ago

Let's start with the basics:

DPF stands for Diesel Particulate Filter. It is a filter in the exhaust system and its job is to scrub soot from the exhaust gases and store them in a canister.

DEF stands for Diesel Engine Fluid. This is a urea-based fluid that is stored in a tank and usually has a filler next to the diesel filler. This system works similarly to the DPF system and also scrubs soot and stores them in a canister in the exhaust system. 

Both systems will then burn off the excess soot by going into regen mode.

EGR stands for Exhaust Gas Recirculation and this system takes a portion of the exhaust gas leaving the combustion chamber and redirects it into the intake system. The system does this to re-burn the exhaust gas and clean out more pollutants.  

All of these systems are part of the emissions systems in late model diesel vehicles and are required for road-legal vehicles.

What are the downsides of these systems?

Both systems have dirty exhaust gases flowing through them and are bound to get clogged by deposits. The DPF system being clogged will cause your truck to go into regen mode, which limits the power and speed allowed. The engine will then inject diesel into the chamber to super-heat it. (There have been fires reported from a truck in regen mode being parked on farm fields.) 

Both these emissions systems add extra heat into the exhaust and combustion chamber. The extra heat will reduce power and increase fuel consumption. 

Excessive idling of a diesel engine will cause these systems to clog faster. People who use their trucks for agricultural or race applications don't need to meet emissions standards. They would consider deleting their systems as a solution. This also comes along with the benefits of an increase in power, an increase in fuel economy and better reliability with lower maintenance costs.