Most backpackers won't have the opportunity to carry a 75+lb pack because their trips are either short or their resupply opportunities ample. Their packs COULD get heavier if they want to include more comforts items and a heavier shelter etc etc. Books. Weird fruits like cantaloupe or avocados, even a Watermelon AGHAST.
But BuckeyeBill brings up a good point---the benefits of training with significantly more weight than you will be carrying on the actual trip. Part of backpacking is all about Managing Discomfort---and a big part of backpacking is dealing with long days, tired legs, sore tendons, a sprained brain, cold and exhaustion---Oh and with pack weight on your back.
One way to duplicate such distress IN TRAINING is to carry more weight---in training.