A bit of historic background
Before Mathematica version 6 graphics were produced as a side-effect much as Print works now. In fact you can load this old system using:
<< Version5`Graphics` Now you get this behavior:

Note that the output is - Graphics - and the plot itself is handled like Print. Since there was often little value in having - Graphics - output it was common to see plot statements terminated with a semicolon, which returns Null which is not writtenwhich is not written to the Notebook.
In version 6 syntax highlighting of that trailing semicolon was added to remind people that with it no output at all would be produced, since the Graphics object is no longer printed as a side-effect.