Why does the universe need quantum superposition? My question is not about the nature or reality of quantum superposition, superposition is a fact and no argue about that.
My question is: What is the purpose of superposition and why does the universe need it?
Every law of physics has its necessity, and makes perfect sense that the universe cannot work without it. But when thinking about superposition, it seems physics laws will work fine with or without superposition!
Edit: I am not questioning superposition itself, but the wisdom behind it. Imagine a world without superposition, where every state is determined from initial conditions, what will change in the universe?
 A: Your claim that

when thinking about superposition, it seems physics laws will work fine with or without superposition

is pretty much completely incorrect, and there are plenty of experiments that can only be given a consistent interpretation using quantum superpositions. You can take your pick from just about all of quantum mechanics; my favourite is the experiment in the second half of this answer, but you can equally well consider Mandel dips, atomic clocks, spin echoes, every single experiment where entanglement plays a role, the structures of every atom and molecule and indeed the basis of all of chemistry, and a myriad others. 
If you want a list of all the experiments that need superpositions to be explained, though, this is not the venue for it.
A: 
Why does the universe need quantum superposition?

Because of the wave nature of matter. Check out the Wikipedia quantum supposition article: 
"Quantum superposition is a fundamental principle of quantum mechanics. It states that, much like waves in classical physics, any two (or more) quantum states can be added together ("superposed") and the result will be another valid quantum state; and conversely, that every quantum state can be represented as a sum of two or more other distinct states. Mathematically, it refers to a property of solutions to the Schrödinger equation; since the Schrödinger equation is linear, any linear combination of solutions will also be a solution".
Note that the Schrödinger equation is a wave equation. We can diffract electrons because of the wave nature of matter. And neutrons too. And protons. Check out Direct Measurement of the Quantum Wavefunction by Jeff Lundeen and others. He reckons wavefunction is something real that can be measured in the lab. 

My question is not about the nature or reality of quantum superposition, superposition is a fact and no argue about that. My question is: What is the purpose of superposition and why does the universe need it?

It doesn't need it per se. And it doesn't have a purpose per se. That's just how the world is. Everything is fields and waves. That's what quantum physics is all about. These waves can "ride over each other" or intersect or combine, whereupon they're in superposition. 

Every law of physics has its necessity, and makes perfect sense that the universe cannot work without it. But when thinking about superposition, it seems physics laws will work fine with or without superposition!

These laws don't have any real existence in their own right. We observe things and take note of evidence, then we come up with theories that include "laws" that describe how the universe works. 

Edit: I am not questioning superposition itself, but the wisdom behind it. Imagine a world without superposition

I'm afraid I can't. Because I can't imagine a world without waves. A world without waves isn't much of a world at all. 

where every state is determined from initial conditions, what will change in the universe?

I can't say that I know for sure. But I fear the answer is nothing. A world without waves isn't much of a world at all, and nor is a world without change.
A: There are many physical phenomena which cannot be explained without superposition. But I won't go into that since it is pretty obvious. I would like to state another rather abstract reason to this. First of all, obviously, the question is a bit too vague it would be really helpful if you were to state it eloquently. 
Anyway, so, most (I think it is quite safe to say 'all of') of the fundamental laws of physics are described by differential equations. And until we are talking under small angle approximation we see that if an equation has a solution x1 and another solution x2 it immediately follows that x1 + x2 is also a solution of this equation, this latter solution is known as superposition of the first two solutions. So, now, my argument establishes that superposition is indeed something which follows from the fundamental laws of physics. (For example, Newton's Laws of Motion and Schrödinger Equation in Quantum Mechanics; both of these are second order linear differential equations) Also you wouldn't see this happening in Nonlinear Systems or when you're out of the small angle approximation. 
Now, I would also like to add something philosophical about the term 'superposition' you see superposition is not something physical like an Electromagnetic Wave or an Eectron or the Earth or Force or Energy etc. It is not even a phenomenon but it is a description of a certain kind of state of a system. 
