The most important advantages of the Spaceships Ferry Voucher are the fixed price throughout the year and the flexibility it offers - send in your ferry sailing dates later and once a ticket is issued it'll be a flexible ticket so you can make ferry date changes for free.
As the ferry companies use flexible pricing throughout the year, they are sometimes cheaper but often - especially in the busy season and at peak times - they are more expensive.
When comparing prices make sure to check the ferry companies' flexible ticket price as that is what you'll get when you add a Spaceships Ferry Voucher to your booking: A FLEXIBLE FERRY TICKET!
See also: How to turn your Spaceships Ferry Voucher into a ferry booking.