This is useful when you have more than one program that will open the same file (format) types.  For example, Microsoft Edge and Adobe Acrobat DC will both open PDF files.  Setting the one you prefer as the default will make working with the files more convenient.

1. Click on Start (windows icon on bottom left of screen)

2. Click on Settings (The gear icon)

3. The Windows settings window should appear.  Click on Apps.

4. Click on Default Apps on the list on the left side.  On this page, you can update your default apps for mail, web browser and video files if you choose to.