Kawa 「河」 is formed by the discarded tiles. In Japanese, it literally means "river", and it can also be referred to as the "pond". Regardless of reference, the tile discards provide players with different information regarding the current state of the hand. For defensive play, players rely on the discards to determine safe tiles. They can also be used to determine probable hands of other players, especially when particular yaku are sought for.
In most variants to mahjong, discarded tiles are simply placed in a disorganized pile at the center of the playing area. However, for Japanese mahjong, each player is in charge of maintaining one's own discard pile. The customary rule of etiquette suggests discarded tiles to be arranged in rows of six, but it isn't mandatory under casual settings. Under more formal settings, the customary rule may be required.
The furiten rule is the primary reason for the arranged discards. Without this rule, then theoretically, Japanese mahjong would not require arranged discards. The arranged discards helps maintain a record of player discards, which makes the enforcement of furiten possible.
The arranged discards provide players with a wealth of information regarding the state of the hand, as well as the game.