Why usually most of the leaves are removed about 1 week before harvest?

There are no benefits only drawbacks with defoliation, in my opinion. Any leaves you remove, no matter which stage your plant is in, will just reduce the plants ability to make energy, thereby slowing growth/maturation. The older leaves also contain valuable "pre-made" elements, such as carbs, sugars, amino acids etc, that your plant will draw upon during flowering. It is far more efficient for your plant to "re-purpose" these "stored in the leaves" elements, than it is to grow flowers and make these elements from new during the flowering phase. This is why plants can go yellow (the fade) during flowering, even though the plant is otherwise healthy and well fed. So, not only does defoliation reduce your plants ability to make energy, it also robs her of a vital source of pre-made vital growth elements that she has so cleverly stored in her leaves for use during flowering. In my opinion, only yellow leaves should ever be removed, for by the time they are yellow, they have served their purpose. The worst time to remove leaves is during flowering, due to the reasons given previously. Besides, if your plant did not need those leaves, it would not have grown them in the first place! Also, your plants' flowers will dry more evenly and with healthier cannabinoids/terpenes if dry trimmed and not wet trimmed. Just remember - Maximum amount of healthy green leaves = maximum energy produced = maximum growth and maximum health. Hope this helps,......... Organoman. (and dedicated anti-defoliation advocate!)