Not all of this information is needed all the time. That's where RNA comes in. Dr. Quatrano has mentioned a couple times that plants can't run away from danger or hardship. They have to make do where they are. So let's say that a plant is in an area suffering from long-term drought.
Most plants react to lack of water by spreading more roots to try to find deeper water sources.
DNA tells it how to do that. The promoter unwinds the section of DNA that codes for, perhaps, the production of a root growth hormone. The two strands of DNA separate to allow RNA to do it's work. You can think of DNA as a series of business letters. RNA looks for the start of the letter with the instructions it needs for a particular job.