Why wait for water to boil?
The primary reason is for accuracy and reliability in cooking times. Boiling water is guaranteed (not accounting for altitude) to be at 212 F (100 C). With a set temperature you can then say things like "boil X for 9 minutes" with a very high measure of confidence.

You certainly can cook things in the water as you go, but it's going to be a lot more hit-and-miss. Pasta, for example, will begin to cook before it reaches the boiling point. You would need to measure the temperature, and check the pasta regularly to determine when it was done. You can't give someone an accurate cooking time with this method either. The time it takes for water to boil will vary greatly with stove heat output, pot size &amp; shape, and the amount of water.