For more information on why you might want to use R, click here and to see some examples of R graphics, see the R plot gallery, which also includes the accompanying R source code used to generate a variety of different plots.
According to John Chambers, in his excellent book
...to enable the best and most thorough exploration of data possible.
and the associated prime directive is that
the computations and the software should be trustworthy: they should do what they claim, and be seen to do so.