The practical difference
- Buy Now sends only that one line to checkout. The rest of your cart is left alone and is still there afterwards.
- Add to Cart lets you gather several products and pay once instead of paying separately each time.
- A cart can hold products from more than one seller. Each seller then delivers their own line separately.
- A coupon is checked against exactly what you are buying at that moment — a cart coupon does not carry over to a Buy Now order.
If a coupon has a minimum order amount, that minimum is measured against the lines actually being bought, not the whole cart.