They're useful when you have conflicts. For example, if you have types NamespaceA.Jobber and NamespaceB.Jobber, and want to use them both in the same class, you won't be able to just add using statements for NamespaceA and NamespaceB, because then the compiler won't know what you're referring to if you type Jobber. In this case you'd give an alias to one or both of the namespaces.
This can make your code clearer, especially if the namespace is long, because the alternative is to have the whole namespace written out each time you use a type.
