At Rank 1, your maximum carrying capacity is increased by 5 per Rank.
At Rank 2, your maximum Battle Weight is increased by Rank.
At Rank 3, when you would lose items upon Death, you instead lose only half that amount.
At Rank 4, carrying around a lot of stuff has made you tougher, increasing your STR growth by 5%.
At Rank 5, your Item Belt size is increased by 2.