You can save pixel selections for later use in the current project or future projects. It's useful to be able to reinstate more complex selections at any time and stage of your project.
Any number of selections can be stored as spare alpha channels in the Channels panel for future use. From there you can reinstate any selection on demand. To learn more visit Pixel selections from channels.
The selection is stored at the bottom of the Channels panel as a new 'Spare Channel' entry.