There are several significant differences between a used product and a refurbished product.
A used product is a device you purchase from its previous owner. Its working condition is not guaranteed, it does not come with a warranty, and it could come with user data that wasn't deleted. In essence, you are buying someone else's device with very little knowledge about its performance and history.
A refurbished product is a device you purchase from a professional refurbisher. It has been examined by a certified refurbisher, who has replaced or fixed any faulty part. A refurbished device is in full working condition, comes with a warranty, and offers the same user experience as a brand new device. You are buying a device that is as good as new, with guarantees as to its performance.
As you can see from the handy diagram below, a refurbished product comes very close to buying a new Apple product.
When you buy a refurbished device, you know exactly what kind of cosmetic condition you can expect (there are different grades for this) and you can be confident that you are getting a device that is in full working condition. You are also not creating any more electronic waste (the planet says thanks).
And did we mention the awesome refurb discounts?