Why would anyone buy a refurbished iPhone device?
One of the reasons to buy a used phone is sustainability. You’re giving the iPhone a second life. Usually, they are cheaper as well. When you compare a brand new one with a refurbished device, you’ll see you can save quite a lot.

So they are cheaper, but do we see an increase in refurbished phones on the market?

The market share for used phones is not very large yet, but it is increasing. If you compare it with the market share for new phones, there is still a lot to win.

I would consider to buy another phone when my battery isn’t working properly anymore. How do they manage the battery at used phones?

Companies that are allowed to use the Refurbished Certificate promise that the battery capacity is at least 85%. For the Certificate, the devices are tested at a minimum of 50 points. The list of points to be checked has been compiled by Techniek Nederland and a monitoring and auditing agency; TÜV.

Is it, according to the guidelines of the Refurbished Certificate, stated somewhere that the devices must contain original Apple parts?

As far as I know, there are some companies* that claim that they are using Original Apple Parts, but it is not a part of the Refurbished Certificate. There could be parts inside the devices that are not certified by Apple.