Why might it be desirable to do this? Click here for the answer.

Labeling your plots makes them clearer to those who have to read them. By itself a plot is little more than a line inside a rectangle, it is the numbers and labels on the axes, and the overall description of what is being plotted, that give a plot the power to convey a lot of information in succinct fashion.

To learn more about giving your plots labels and names, read the Mathematica Book, page 418 ff. where there are many descriptive examples.