Here are the 4 main benefits of using a vertical mouse:

Keeps the forearm and wrist at a neutral rest position, which minimizes muscle use and arm pain from sustained pronation. Avoids forearm and wrist pronation (rotating of the wrist counterclockwise) that compresses the median nerve and can lead to carpal tunnel syndrome. Keeps the bulk of the underside of your wrist off the desk, further reducing pressure in that area. The handshake position lets you operate the mouse with your forearm instead of the wrist if you choose. Great for people with existing carpal tunnel syndrome or wrist strains. As you can see, a good vertical mouse can not only help in the prevention of CTS, Tendinitis and certain musculoskeletal diseases but can also enable people who already suffer from these conditions to continue using a mouse without exasperating their condition.