When using reset() the parameter passed to reset need not be a managed object (nor can it be); whereas with = the right hand side must be a managed object.
So these two lines give you the same end result:
p = std::make_shared<int>(5); // assign to a newly created shared pointer
p.reset(new int(5)); // take control of a newly created pointer
But we cannot do:
p = new int(5); // compiler error no suitable overload
p.reset(std::make_shared<int>(5).get()); // uh oh undefined behavior
Without reset() you would not be able to reassign a shared pointer to a different raw pointer without creating a shared pointer and assigning it. Without = you wouldn’t be able to make a shared pointer point to another shared pointer.