Here's a couple of examples of why we believe in the concept of time--memories and predictions.
If we didn't remember that it hurt when we touched a hot stove, we might do it again. And say we were still hunter/gatherers--if we didn't plan to be at a certain place when the turnips ripen we might starve. Even in the modern world, if I didn't get up and go to work now, later I wouldn't get paycheck and then I wouldn't be able to afford food and shelter. So it has to do with survival of the species. Lots (all?) of our behavior does.
At first, I thought being in the now meant rejecting the past and the future, but you can think about the past and be present or plan for the future and be present.
I know that time does not exist the way we think of it, but, still, if I want to see the Simpson's Movie tomorrow, I better know what time it is and keep an eye on the clock!