Photon: System.Collections.Hashtable vs ExitGames.Client.Photon.Hashtable

I am working with Photon in Unity now, and in all tutotials I can see that people use ExitGames.Client.Photon.Hashtable to set customRoomProperties. My question actually is: what are the difference between default C# Hashtable class and photon Hashtable class. And why I should use only Photon Hashtable to set my room properties, if they are both representing a KeyPairValue type?