As to your first question why do plants need phosphorus, I agree with Kris. Plants need phosphorus because it helps them convert nutrients into the building blocks they need to grow. This is how they make the food they absorb into something useful.
As to your second question, I think you can try these methods:
One way to enrich phosphorous levels is with chemical fertilizers. If you choose to go this route looks for a fertilizer with a high P count. For instance, if the bag says it has a fertilizer rating of 10-10-10 then it has equal amounts of nitrogen, phosphorus, and potassium. If, however, it reads 10-15-10 it contains more phosphorus than it does the other two nutrients. Phosphorus is always the second number.
You may also use organic methods to increase the phosphorus level. Sometimes just adding compost to the soil is enough to help the roots take in more of the available phosphorus but if not, try working in some rock phosphate or bone meal around each plant.
You do want to be careful not to overdo the phosphorus, though. Not because it will damage the plants. It’s almost impossible to give a plant too much phosphorus. Excess phosphorus is a major pollutant, however, and excess will run off into the water supply.