Adobe Illustrator is the most important software of digital design word (especially vector base design). Yes Pen tool is the most important tools of Adobe Illustrator. Without properly using pen tool, it is impossible work with Adobe Illustrator. It is very easy modify an objects which create using pen tool. I am working on the word famous market place “upwork” (previous Odesk and Elance). I prefer convert image raster to vector for this purpose pen tool is more useful.
I think without Pen Tool Illustrator is not fulfill its goal. If you want create an object free hand, You need Pen Tools. For the same way, If you want create a path need Pen Tools. Here is a youtube video Learn How to use Pen Tools.