Question

why should value working in stack and reference managed in heap?
Ref---value contain null
reference contain null

Answer 1

Not sure what you're asking. Generally whether a value is allocated on the stack or heap is an implementation detail. Things allocated on the stack are generally faster to allocate than things allocated on the heap, though.

Answer 2

One important difference is that stack-resident values have a lifetime only as long as the function/procedure is running, and they literally disappear (they "go out of scope") when the function exits. Heap-resident values can be useful for the entire running of a program of multiple functions.