If you only know one method of attachment for example, what do you do when you switch from worm fishing with gang hooks for trout to fishing a crank bait for bass? In this scenario if you only know one fishing knot you are more than likely using a knot that it much less strong than it should be in one fishing situation or the other. There simply isn't one "magic knot" that will work in all fishing situations, although many anglers tend to think this is the case.
Tying fishing knots is an extremely important skill to have as an angler and knowing at least three different methods of attaching line to tackle is of the utmost importance in my opinion. In reality an angler should know more than three, but three will suffice for most fishing scenarios.