If you want to reconnect to an old session after e.g. a network failure.

If you set the SessionReuse property to true, the UA Client connects to the old session if possible when reconnecting, for example, in the case of a network failure. If a client loses connection to the UA Server (e.g. OnShutdown is fired) and you want to reconnect, the client tries to connect to the old session. This behavior is useful, for example, if you have an OPC UA server with a limited number of sessions. 

See also: https://reference.opcfoundation.org/Core/Part4/v105/docs/6.7 

By default, this property is set to false. This means that after e.g. a network failure a new session will be created. 

To detect if an old session has been reused after reconnecting to the server, you can use variable SessionWasResued.