The hardener adds some gloss to the Rusto, helps flowing, makes a more durable finish and yes, dry faster. Rusto can take quite a while to fully cure and adding the hardener greatly decreases the cure time. Once a batch of hardener treated Rusto is mixed, you have a max of 8 hours to use it and toss the leftovers.
You don't have to use hardener. But if you like to sit around watching paint dry................:rolleyes. If you paint with straight Rusto and let the boat sit for like a month, the results will be about the same except for the gloss.