Old fixtures in many homes don't meet these standards. If yours don't, you can save water and money by replacing them.
These standards are now a bare minimum though.
There are many fixtures on the market that exceed the standards, some by quite a margin, yet perform quite well. Some showerheads deliver excellent showers with only 1.0 to 1.5 gallons per minute (gpm). For your bathroom faucet, a 0.5 to 1.0 gpm model should work well. You'll probably want to stick with 2.0 to 2.5 gpm in your kitchen sink, where a higher volume is necessary to fill pots and such. For toilets, there are models available that work very well with .8 gallons per flush for liquid waste, and HET, or High Efficiency Toilets at 1.28 gallons per flush, or for some toilets, 1.6 gallons per flush (gpf).
Unfortunately, some earlier 1.6 gpf toilets didn't perform acceptably. Some early versions of these "ultra-low-flush toilets" introduced in the 1980s and early 1990s were particularly problematic. Customer surveys by major utilities that have sponsored toilet replacement programs have uniformly shown that most consumers with the newer toilets are very satisfied with their new toilets.
Dishwashers, clothes washers, and other water-using appliances aren't subject to any national water-efficiency standards in the United States (although they do have to meet energy standards that tend to favor water efficiency).
Nevertheless, the major manufacturers now make some highly water efficient washing machines and dishwashers. These models are definitely worth considering because they provide triple savings. First, they save water. Second, by using less water they save on water-heating energy. And third, they often require less detergent because the volume of water into which the detergent is dissolved is so much less. For clothes washers, look particularly for a Front Loader, or a "horizontal axis" model. These washers use one-third to one-half as much water, energy, and detergent as traditional top-loading models.
The speed with which new fixtures and appliances are becoming available makes it hard to keep track of them all. Misinformation abounds. A local plumber or hardware store may be able to advise on water-efficient products, but most aren't up to speed on this subject. The resources below should help you find products that save water and satisfy your performance needs.