Kawa
Kawa 「河」 (lit. "river", sometimes referred to as the "pond") is the discard pile. It helps provide players with information regarding other players' hands. For defensive play, players rely on the discards to determine safe tiles. It can also be used to determine probable hands of other players, especially when particular yaku are sought for.
In most non-Japanese variants of 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.
Furiten
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.
Discard reading
The arranged discards provide players with a wealth of information regarding the state of the hand, as well as the game.