What Good Is a Mobile App?

Serious question...

What tasks are mobile apps uniquely good at accomplishing? Why would an organization turn to building a mobile app rather than using a desktop application, email, or web-based application? What can a mobile app do well that nothing else can?

This is an important question for anyone trying to sell a potential client on the need for a mobile app, or for anyone who wants to design an app people will download - an in-demand app should include features people can't get elsewhere.

So what needs can only be satisfied with the creation of a mobile app?

Please post your thoughts! Thanks!