No office space required. No additional equipment required. No benefits packages to worry about. You get well-trained and experienced help when you need it. This makes a tremendous difference in her attitude about your business, and how your work is done.
Most temp agencies charge as much or more than a virtual assistant does.
They have to, in order to pay their best temps well and still make a profit. Unfortunately, you will probably have a hard time getting the same good temp whenever you want her. You won't have that problem with a virtual assistant. The virtual assistant is an independent business person. She pays her own taxes and provides her own office and equipment.