Imagine for the sake of argument, that we have 9 candidates, and one, "Bob", is a clear favorite. I may not hate Bob, but I really think that "Jane" would be much better. I might even consider Bob a distant second choice.
I could "vote my mind" and say Jane #1, and Bob #2, as the voting system would have us do. Or, I could think that it's in my best interest to rank Bob very last, knowing that he will probably win anyway, as a way of hoping to push Jane to the front, so I vote for Jane, #1, Lousy candidate #2, and really lousy candidate #3