Low flow water fixtures save water, energy and time for you. As the name suggests, the low-flow fixtures restrict the amount of water flowing out of the water fixture, to an optimum and comfortable level to prevent water wastage. For example if you had a "fire-hose" equivalent of a fixture (non low-flow), you would have more water coming out of your fixtures than required or needed.
Low flow fixtures are a “green’ and an environmentally friendly strategy, and is recognized by all green home rating agencies ( LEED and GreenPoint).
Low flow water fixtures are "green" and save water, energy and your time
Low flow water fixtures such as the low flow bathroom and kitchen sink, and low flow bathroom shower heads, can put less strain on a tankless water heater (amount of hot water they can supply simultaneously). Low flow water fixtures/faucets and aerators limit the water coming out of the faucet to an efficient yet comfortable level. There are also low flow plumbing fixtures available in the market.
US EPA's (Environmental Protection Agency) site on low flow water fixtures and 'WaterSense' label products, is a great resource on further learning on low flow fixtures and their advantages. EPA has also introduced 'Water Sense' low fixture standardization and labeling for easy identification of low flow fixtures.