Oh! I got confused by that too, initially. It says this though:
"On a 4 or lower, it’s a weak hit: you get what you want, but there’s a cost or a complication. You can choose to roll +1d6. If you do so, you MUST add it to your result."
So if you rolled a 3 and don't want to deal with the complication, you could choose to roll an extra d6. If you roll a 1, then 1+3=4 and you'd still have to deal with the complication. If you roll a 3, then 3+3=6 and there's no complication. If you roll a 5, then 3+5=8 and you not only have a complication, but you don't even get a hit. So it's a total push your luck game, giving you some extra control even after the die is rolled.