If knowledge is defined as justified true belief, why is it necessary to include
Different philosophers would answer this sort of question different ways, depending upon how they approach epistemology. On one sort of view, we might think that this is a question about the meaning of the ordinary English verb "to know". And in that case, there seems to be good evidence that English speakers are not always willing to describe someone as "knowing" something just because they believe it and it is true. For example, the Super Bowl is tomorrow. I know nothing about football, but suppose I firmly believe that the Seahawks are going to win. If they do win, would you want to say that I knew they would?
A more interesting question, though, to my mind, is simply whether we think there is an important distinction to be made here, whether or not it is one that is made in English. And that, obviously, has to be decided by seeing what work "knowledge", in that sense, can do. It's not at all clear, actually, that all one's true beliefs can "successfully guide...activity in the world" in the same way, whether or not one has justification for them. Timothy Williamson has some nice examples along these lines in his book on knowledge, though he would deny that knowledge can be reduced to anything simpler.