What is the Rectangle tool good for and what is its keyboard shortcut?

The Rectangle tool is used for drawing rectangles and squares onto the Artboard. The keyboard shortcut for using the rectangle tool is R.

In order to draw a square, hold the Shift key pressed while you draw the shape.

Each rectangular shape can have different

• Size,
• Opacity,
• Fill (Solid color/Linear gradient/Radial gradient), 
• Border,
• Shadow and
• Background/Object Blur.

The shapes are the basic Design building elements.