In order to fully understand the results of their experiment you have to understand their method of labeling. There were to batches, one labeled with radioactive sulfur and the other labeled with radioactive phosphorous. Their reasoning for the use of the two different radioactive elements was based on the structure of and composition of the two objects being labeled. The reason that the DNA inside the heads of the phages in batch 2 was labeled with radioactive phosphorous is because DNA contains phosphorous but NOT sulfur. The reason that the protein inside the heads of the phages in batch 1 was labeled with radioactive sulfur is because proteins contain sulfur but NOT phosphorous. This was a convenient way of determining what was what through the experiment and worked well to distinguish what was what at the end of the experiment. The picture below depicts the difference between the two batches.
In the end of the experiment the phosphorous labeling and sulfur labeling was helpful in determining whether the pellet, or bacteria, and the liquid, or free phages and phage parts, was radioactive which would determine which of the two substances, DNA or protein, is the true genetic material. In the end, it was found that DNA was the genetic material because there was radioactivity found in the pellet, or bacteria, and there was no radioactivity found in the pellet, of bacteria, of the protein mixture. This relatively simple way of labeling the plausible genetic material was a good example of the high-quality, controlled experimentation of Alfred Hershey and Martha Chase.