The strainer basket prevent waste from entering the impellor housing which may jam or damaged the pump. Whilst it is not entirely necessary to have the basket fitted if you are sucking water out of your filter, it is entirely possible that stray media could find its way it to the pump. If the strainer is not fitted, any damage would not be covered by warranty and as such a repair would be chargeable. If sucking from the pond and pushing water in to the Ultra Violet and filter it is highly recommended to fit a strainer basket.