Vertical Clustering can do 2 things as I know:
Let us take an example: 1 Cluster with 2 application servers running on the same box
1) Protection against software failure: If one of the application server run into problems other server can still take the requests
2) Application update without outage: After updating the Application (Ear), Rolling recycle (One application recycle at a time) of application servers need to be performed. This way one server is available to server the request at any time.