Have you taken any physics classes? Calc based is better for what you want to understand, but not necessary. I think you will have a better understanding of "why calculus works" when it is applied to real things. That's the whole reason math was invented in the first place. Newton invented calculus so he could describe the physical universe.
For example when you want to know what the distance is, given velocity and time, you plot velocity vs time and find the area under the curve (or line); that's an integral. You don't need calculus for that, but calculus lets you do it without the graph. The instantaneous slope of that curve/line is the acceleration.
ie. plot a simple graph with the data:
V = 10, 20, 30, 40...etc
t = 1, 2, 3, 4....etc
Now, when you graph it, you'll get a triangle. Find the area of the triangle using geometry. Next, set it up as an integral.
V(t) = dD/dt ; D is distance
dD = V(t)dt find the function V(t).....it's V(t) = 10t
Evaluate the integral for whatever time limits you used on your triangle. You will get the area of the triangle.
There are some good math history books that go into great detail as to how Newton and/or Leibniz came up with it and how/why it works.
Seems like the better question is HOW it works as opposed to WHY. WHY is because that's the way the universe works. Or, that's how God made it.