It is a matter of safety, convenience, and pride in seamanship to be able to quickly tie important knots and bends as well as decorative knots used throughout the ages on seagoing vessels.
Some boaters even consider knot tying a method of relaxation.
Send your crew to this class if you're the type of skipper that never seems to take the time to get your crew proficient in this important part of boating. If you wish to take your knots home with you bring your own rope.