Muscle cells use creatine primarily to form creatine phosphate, a high-octane chemical which provides the energy needed for short, fast efforts. Creatine phosphate does this by 'donating' its phosphate to a chemical called ADP; this forms ATP, which provides the energy for all work done by muscle cells

In addition to supplying energy, creatine phosphate also acts as a 'buffer' which helps to downgrade increases in muscle and blood acidity during exercise. Creatine phosphate also carries energy from the part of the muscle cell where it is created to the part where it is actually used to make the muscle shorten. This energy-carrying process is absolutely essential for normal muscle function during exercise

Sodium phosphate appears to raise 2,3-DPG levels in red blood cells, making it easier for those cells to release oxygen to muscles (this is probably why phosphate supplementation has so often been linked with higher maximal aerobic capacities). The oxygen is then used to create energy, and the energy is carried to the right spot in muscles by creatine. Thus phosphate and creatine make a great, performance-promoting tandem

Since phosphate boosts oxygen delivery to muscles, the heart doesn't have to work as hard during exercise (after all, its main job is oxygen delivery). That allows athletes to run at quality paces at a smaller percentage of their maximal heart rate, which gives them more cardiac 'reserve' for speedier running. Phosphate itself is also a buffer, and of course ingested phosphates can be used to form creatine phosphate and ATP, the important chemicals mentioned above.