Pencil/Paintbrush vs. Pen

They all create paths that when viewed in outline view look the same. The pen and pencil tools are two different ways to create the same thing. With the pen you manually put the points and control the shape of the curves with handles. With the pencil you draw like using a real life pencil and based on your movements and speed Illustrator adds the points automatically and creates the shape of the curves according to parameters that you can set in the options of the tool. You can start creating a path with one tool and continue drawing or editing the path with the other tool. Also the Smooth and Path Eraser tools under the Pencil group in the tool box are basically the pencil tool in special mode.

The brush tool functions and is basically the same like the pencil but the only difference that makes it unique is that it can react to pressure sensitivity input which creates variable width of the stroke and is built into the path. This pressure sensitivity appearance cannot be edited or reused on other paths. You can apply a brush appearance to any path being drawn with any tool  even during its creation by clicking a brush in the brushes panels. This doesn’t change the current tool but applies the brush appearance to the path.

Another way to create variable width stroke is using the Width tool which can be used on paths without pressure sensitive input. The appearance created with the Width tool can be edited and reused on other paths using the Profile menu in the Stroke panel.

You can remove the variable width and pressure sensitive brush appearance from paths as you have already found out by playing around.

Just experiment, play around and  you will discover what tool is most suitable for certain tasks. I use all of them depending on the job. But naturally the pen and brush tools are best used when using digital pen. I don’t think I would ever use these tools if I have to use the mouse. I is like in real life trying to draw by pushing and dragging a brick.

There are reasons youyu might use the paint brush and the pencil tool rather than the pen tool and experience will tell you when the pen tool is simply more versatile and accurate so it has more usage but say you wnted to fill an object with a brush like texture like such  the brush tool will work much better than the pen tool. Circumstances usually dictate which tool is best.

Probably personal preference and whatever the project calls for.  I like using the Pencil tool when I want somewhat a hand drawn look.  The Pen tool is more accurate and technical.  Problem I find is the Pencil tool leaves you with a lot of extra anchor points.  The Brush tool is pretty close to the Pencil tool with a few extra options.