Possible scenarios to justify heap-allocated variables scoped locally?

Solution 1:

First of all there is no reason to not use a smart pointer here, i.e.

auto a = std::make_unique<A>(/* arguments */);

but as to why you would want to heap allocate rather than creating the object on the stack, reasons include

1. Size. Class A might be huge. Stack space is not inexhaustible; heap space is much much larger. Seriously, you can overflow the stack surprisingly easily, even on modern machines. You don't want to stack allocate an array of 100,000 items, etc.
2. You might need a pointer for runtime polymorphism. Say instead of calling A's constructor directly you are calling some factory function that returns a unique_ptr to a base class from which A inherits and the rest of your code depends on polymorphic calls to a; you'd need to use dynamic allocation in that case.