Why does the STL reserve an interface for Allocator?

Question

Why does the STL reserve an interface for Allocator? Taking vector as example:

template<class T,class Allocator = std::allocator<T>> 
class vector;

Since there are many options for us to allocate memory and construct objects like
operator new,delete,new[],delete[],which could do almost anything we need when we create an object.
So why does the STL containers like vector need an Allocator interface,which at most time is the default std::allocator if we don't assgin one? Why not just use the new expressions?
If the purpose is to make user-defined allocating behaviors possible,why not just let the users provide their self-defined operator new,new[],etc?

Answer 1

So specific vector and change how it allocates without changing how all vectors allocate.

new must be overridden on a per-type basis of what you are allocating, or replaced globally. std::vector<int> and std::vector<int, some_other_allocator> can use a different allocation strategy; in fact, the allocator can be stateful and have shared state.

A classic one is a fast stack allocator that doesn't deallocate until it goes out of scope.