The basic principle of lightweight backpacking is to carry the bare minimum of weight. The less weight you're lugging up hills the further you can travel and the more enjoyable it will be.
Rufty-tufty squaddies and callow youths walking for badges may wish to get their bragging rights with 'my pack's heavier than yours' but for the discerning, ahem, older walker the absolute minimum grammage is where the real kudos is to be found.
Obviously you can take nothing, beyond money, and stay in B&B's, eat take-aways etc but then you'll miss out on camping in your tent deep in the countryside and eating great food gazing at a perfect view (if it's not raining).
The real 'racing snakes' can pitch, scoff, kip, pack-up and carry on with their mega-mile outings with the minimum 'time lost' using this system.
Those who appreciate life a bit more can make themselves very comfortable with their lightweight kit, kick-back, and watch the young speed by while savouring great food that weighed next-to-nothing, was perfectly 'cooked' when required and involved no washing up.
Lightweight is the new black for walkers, the weight of efficient, reliable kit has reduced drastically in recent years. What appeared to be lightweight 20 years ago is now labelled as 'car-camping' gear and everyone compares grammes and ponders how to shave weight from their kit.
Even food hasn't escaped the revolution.
'Wet' food is about 50% water - not counting the packaging.
1 litre of water weighs 1Kg, some 1-man tents are only 750gm!
Plus whisky weighs the same as water so less water carried means the more 'wee drams' you can sneak into your pack.
The countryside is full of free water, sometimes it comes to you, other times it's in nearby streams etc. Obviously sensible precautions regarding which water you use & purifying it before use are important considerations but it makes no sense to carry extra weight as water in your food when there's loads of it lying around.
Far more sensible to carry your food minus the water and then replace it when you want a meal.