WHY USE HSTACK AND NOT VSTACK

While trying to stack the existing loan-data-numeric file with the new exchange_rate column data, since we are adding a new column, I was hoping it should be vstack that we will use. Can you please shed more light on the technicality behind why we had to use hstack inlieu of vstack.

And when we wanted to add the heading too, why dont we just use the either of the stack functions? But, rather using the concatenate function?

Thanks.