Dedicated extensions make your organization much more navigable by your potential callers. With the ability to establish extensions, you can allow your clients to contact specific departments within your organization or individuals within them.
This is traditionally very useful for hospitals and schools, which have many different departments often helmed by a specific person who may need to be contacted individually.
In the modern business age, international corporations hugely benefit from the extension dialing feature as it allows employees to easily contact their department heads or counterparts wherever they are in the world. Being able to dial a company contact without having to find the specific area code is an invaluable time-saver.