That’s s a very good question, with multiple answers.
The truth is that O.E. manufacturers spend big dollars to save vehicle weight. Personally, weight savings is something that appeals to me. Every 100 lbs saved allows you to reallocate that weight to something else. Remember that you only have so much GVWR to work with. Capacity is an important factor to keep in mind for handling, and fuel efficiency. Weight savings benefits you in many ways; because of the lightened load your engine doesn’t have to work as hard, in turn saving you money all around (even at the pump)
Less weight equals less stress on the engine, drive train, suspension, brakes. Sprung and un-sprung weight means longer life.
Modifications for off-road suspension and tires also affect gas mileage. All-terrain tires produce less friction and therefore roll more freely. They’re generally lighter than pure off-road tires, and less weight means better fuel economy. They also ride quieter, handle better and wear longer. New tires are among the first changes an off road enthusiast makes.
It’s been said that every 100 lbs you add to the vehicle, you need an additional 10 horsepower to maintain the same performance levels. Keep the weight low when you have a choice. If you can shave 100 lbs from what you were going to install and go with lighter weight pieces, you could re-allocate that weight to carrying tools, more vehicle modifications, water, survival gear, spare parts, camping equipment, firewood, etc.