Each cell has its own DNA, which has the blueprint for what that cell is and what it does as a function for that part of the body. The DNA is housed in the cells nucleus, located in a central part of the cell. Think of the DNA and the nucleus as being the "thinking part" of the cell; it tells the cell what to do, how to do, and when to do. It is a very active part of the cell's life cycle, so much so that necessity dictated it be located on site within the cell. Things evolve, often times, out of necessity, so the need to have the DNA, which is different for each type of cell, "local" as opposed to "long distance", seemed to be the path of least resistance.