Selection tools have three uses.
Choose the use you need for your photograph.
You can use a selection tool like a cookie cutter.
For example, let's say you want to move a sky from one photograph to another.
You can cut the sky out with a selection tool.
Go to Use #1 - Cookie Cutter.
You can use a selection tool like a stencil.
A stencil covers up most of a surface.
The paint is applied only where the stencil has openings.
Similarly, when you make a selection, you can make a change only in the selected area.
The area outside the selected isn't affected by the changes you make inside the selection.
You can change the exposure, contrast, color, and so forth.
Go to Use #2 - Stencil.
Scotch-Blue™ Painter's Tape for Corners
This use is specific to the Clone Stamp tool and the Healing Brush tool.
You can use a selection like blue masking tape.
Let's say you're painting a window.
You may use blue masking tape to keep the paint off of the glass.
Likewise, a selection will prevent the Clone Stamp tool or the Healing Brush tool from editing part of your photograph.
Make a selection where you want the above tools to operate.
The tools will work only the area inside the selection.
The tools won't change the area outside the selection.
Go to Use #3 - Blue Masking Tape.