The objective of basketball is to put the ball through the hoop, so at a superficial level it may seem like the net hanging from the rim serves no purpose. But that couldn't be further from the truth. Basketball rules require the netting because it serves an important visual purpose in the game.
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What the Net's For
The purpose of the net is to simply make it easier for players, officials and fans to tell when the ball has passed through the rim. If a ball sails through a net-less rim it might look like an "air ball" or missed shot. The NBA and NCAA rules both state that the net "shall be constructed to check the ball momentarily as it passes through the basket." This means that the net must be tight enough that the ball pushes its way through, giving a clear visual indicator that a basket has been scored.