Basically you only really need it when you want to use two types with the same fully qualified name (same namespace, same type name) from different assemblies. You declare a different alias for each assembly, so you can then reference them via that alias.
Needless to say, you should try to avoid getting into that situation to start with :)
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
