When something is refurbished, it basically undergoes through a process of renewal and update which makes it fit to use in the modern day. 

Similarly, if an iPhone is refurbished it means that it is a new yet old version of a phone which is fit to use for present times. This isn't a bad idea at all if discussed further.

For instance, the fact that Apple is selling these refurbished iPhones themselves and it is not some third party vendor, it ensures quality. If it is Apple then one can be sure that these old iPhone models are bound to be of good quality.

Secondly, this act increases the interaction between Apple and their customers. According to The Verge, this is the second time Apple is pulling off a stunt like this. The last time they did this was in 2007 and it was quite a success. 

Finally, refurbished phones are always much cheaper than they used to be. 

For example, according to The Next Web, a 16 GB iPhone 6S is selling for USD 450. A 6S Plus 64 GB is selling for a mere USD 589. All of these include free shipping as well.

If this is not a good deal, then nothing is, the reason being that the iPhone 6S and 6S Plus are not very old models. Yet when they came out before, they were very costly. 

Now that the 7 took the spotlight, their price has reduced but their model, their function, their usage, and their modern feel is still very up to date. 

Apart from such, all refurbished phones sold from Apple will also get a full year's worth of factory warranty. 

Finally, all the new phones will be sold with new batteries and outer shells. This means the phone will almost literally be as good as new and no one would even be able to say it is refurbished.