Why use the pen tool over the line, shapes, paintbrush and pencil tools?

Dubbed as the most powerful tool in Illustrator, the pen tool is used to create lines, curves and shapes. As a single tool capable of performing the functions of multiple tools with the added benefits of flexibility and post-adjustments, it would most likely be labeled as such.

Similarly to the line tool, the pen tool can be used to draw straight lines and angles.  However, using the pen tool, line segments are drawn by connecting anchor points. These line segments can automatically connect and expand into regular shapes like those created by the shape tool and quite possibly irregular shapes and curves as well.

The pencil and paintbrush tools allow you to draw free-form as you would with an actual pencil on paper. Unlike the pen tool, anchor points for the pencil tool are automatically positioned and the paintbrush tool does not have anchor points which limits editing.

But the most exceptional function of the pen tool is it’s editing ability. Any shape created by the pen tool can be easily amended using the add, delete or convert anchor point tools. 

Source: www.help.abobe.com