"I was born a 4x4 owner, it wasn't a choice, I didn't become one later in life...I was born this way"
One of the most fun (and costly!) parts of having a 4x4 for many owners is adding accessories, bolting-on performance parts, and changing the way it looks. And it can be an endless pursuit: modifying the rig to make it unique and reflect the personality of the driver is an on-going process for many people. As time and budget allows, some people do it as often as possible. It's common for some 4x4 owners to spend each weekend working on their vehicle - it can be an addiction!
Benefits of Modding
Changing your truck or SUV can get expensive, it CAN make your wife or girlfriend feel like a "car widow", and it can make you the object of derision in the neighborhood (when your 4x4 is always in the driveway on jackstands, apparently it makes surrounding home values go down). But you don't have to care what people think - you're not doing this for everyone else. And modding has some definite benefits.
Improved reliability. Some mods are objectively important for safety and reliability of your vehicle. Some of these might fall under PM (or "preventative maintenance") but many times they are vast improvements over the OEM design. One great example of such a mod would be the "Pesky Heater Hose" bypass fix for FJZ80 Toyota Land Cruisers. The PHH fix can totally eliminate one of the most pervasive points of failure on that vehicle, so it definitely improves reliability. However when a bypass is done (or even a replacement with higher-grade heater hose and clamps) it can also be considered a modification.
Enhanced performance. Many times mods can improve horsepower or performance on rough terrain. These mods also tend to be the highest cost. Some examples are re-gearing your rig, adding larger tires, changing the transmission to a beefier one, swapping engines, or altering the air intake in some way. Other performance-enhancing mods apply more to camping or overlanding - such as adding an auxiliary gas tank or installing a dual battery setup.
Added convenience. A mod that just makes life easier is a convenience mod. These also tend more useful in certain circumstances but are just "good to have". They're not usually visible from the exterior of the rig. Some examples of added conveniences are: on-board air, on-board hot water, upgraded electrical with added USB or other plugs, in-dash navigation, in-dash backup camera, better seats, drawer systems, or different windows such as louvre or flip-up side rear windows. Many of the more creative mods are merely for convenience.
Increased value. There are two kinds of 4x4 owners: those who mod and those who keep it stock. But the true enthusiasts tend to do something to their vehicle and the true "pure stock" rig is rare indeed. With the undeniable benefits of some mods (such as replacing old interior lights with newer LEDs on a vintage vehicle) keeping a vehicle pure stock just doesn't make a lot of sense. Old parts wear out. New parts and added functionality make sense. When a vehicle has been kept in good working order, parts updated or upgraded, and the overall appearance of the vehicle kept within reasonable limits of tastefulness, a modded rig is more often than not more valuable than a vehicle that's been kept pure stock. (So don't be afraid to mod - if you do it right. It usually makes your vehicle more valuable when you wish to sell it.)