Having this kind of policy is important for managing expectations of your customers. This is because it lets them know two key things that matter to every consumer:
How much they can expect to pay for shipping
How long of a wait they can expect when it comes to receiving their goods
If it takes too long for a customer to figure out how much you charge for shipping and generally how long an item will take to arrive, the customer will probably buy elsewhere and you'll be losing business.
Providing an easily accessible Shipping Policy that has clear information can help you gain and retain customers.