Because like
@auralia said, some stores like Target will price match
and offer you a further discount off that lowest price. Or, you may have gift cards to that store that were given to you, earned, or purchased at a discount, which you can use to further reduce your out-of-pocket cost. Also, many states now require online retailers like
Amazon to collect state and local sales taxes. And fyi, sales taxes vary based on locality and are not 10% all around.