Plants that can survive through the colder months will provide a wide host of advantages for your pond and pocket. Having plants in your pond yearround will provide water filtration and oxygenation year round. This is especially important in winter, where oxygen may be in short supply to your pond. Additionally, not having to repurchase and replant greenery every spring will save you money and effort in the short and long term.
In addition, decaying plants can cause damage to your pond if they're allowed to stay in the water and decompose. Winter hardy plants won’t die off and therefore won’t deplete oxygen or create extra work for you by having to clean them out of your pond. They’ll also provide beneficial habitat and possibly some food for your fish throughout the winter.