Let say there's an apple on the table. It just lie there, fresh juicy red apple. You plan on taking it, after a moment you take it and eat it. You eat it, and eat it until it is finished. From the moment you took the apple, you have just created a future. And that future is, you have eaten the apple. But what if, you don't take the apple. Then suddenly someone pass by, take the apple and eat it. Therefore no future about the apple obtained. But what if you take the apple, eat it halfway and put it in a fridge, thus this one create an alternate future.
The point is, everything you do have an impact on the future. It might be small but still it is an impact. You didn't take the apple, big deal... meh... but you just have lost a future, its better if you take the apple wasn't it?
Let say you work hard for the exams, you guarantee your marks will be top-notch, lets say 10A+ . So you've made a plan for that; When I get that straight A's, I want to go study overseas. Therefore you have planned it that way. So you do anything to accomplish of what you have planned. So you take scholarships and such... But what if... you didn't get straight A's. You don't have a plan. What will you do, an alternate future has been developed there. And you don't have any plans at all.
Is there such use for Plan B? We often didn't come with any plan B at all, because we are so sure that everything goes on perfectly as planned, but the thing is, we're not omnipotent. We make mistakes, come to think of it, we are the only creature who makes mistakes. Animals don't make mistakes, or do they?
So what if your plan turn horribly wrong, what do you do then...?