There are many reasons why you would want to potentially create an icon from an existing one:
To save time
Ensure consistency in the data being collected (i.e. properties)
Ensure a uniform look and feel.
To get started, simply hover over one of your existing icons in your library and click on the [+] symbol. You will be prompted to the Create a Custom Icon window pre-populated with data extracted from the icon you selected.
Next, just follow the normal process of creating an icon. You can opt to change things such as the image, ports, properties, and quote part as needed.
Here is a quick video to show you how: