There are more than 2 uses.
1. To import the namespace e.g. using System;
2. To create an alias e.g. using connection= System.Data.Common.DbConnection;
3. To wrap the disposing of an IDisposable in a Try/ Finally block to ensure Dispose is called on the IDisposable
IDisposable d = new DisposableObject();
try
{
....
}
finally
{
d.Dispose();
}