What is the use of Global PickList in SFDC?

A global picklist is a restricted picklist by nature. Only a Salesforce admin can add to or modify its values. Users can’t add unapproved values, even through the API.

From Setup, enter Picklist in the Quick Find box, then select Picklist Value Sets.
Next to Global Value Sets, click New.
Enter a label for the global value set. This name appears in Setup, and when users create a picklist based on this global value set.
To tell users what these values are for, enter a specific description of the global value set. This text appears on the Picklist Value Sets list page in Setup.
Enter the values, one per line.
Optionally choose to sort the values alphabetically or to use the first value in the list as the default value, or both. You can’t change these settings later.
If you select both options, Salesforce alphabetizes the entries and then sets the first alphabetized value as the default.
Click Save.

Your global value set is ready to be used in custom picklist fields. You can reorder the values by clicking Reorder.