Spreader mat are used for the growth and development of your root mass within the NFT system.
It is laid in the channels of the top tray before you put your plants on. The spreader mat becomes saturated with the nutrients from the reservoir via the delivery tube and this is when you place your plants on it. The roots then bind into matting allowing it to spread out for maximum root growth and optimum nutrient uptake. It is essential for spreader mat to be used in an NFT system, it is not an optional element.