Howdy, There are things that a trawler absolutely needs.
Examples are things like engine, steering, and hull.
There are things that are darn good to have. Onboard fuel polishing systems also fit into that last category. It's true that in many cases you'll never see any concrete benefit from having and using such a system just as you may never need to use that spare generator belt that you always carry. Further we have the good fortune to enjoy a very mild climate that isn't conducive to fuel degradation. It is still possible to buy bad fuel and it's also possible for you or your crew to instigate fuel problems.