If you have an old thunderbolt 27" apple screen you're definitely happy about it as it's super good and very neat, but it doesn't work with the new macbook pro's with Thunderbolt 3, therefore you'll need this adapter to get the 2 things to play together.

Please note that this adapter does not work with Apple cinema display LED 27", it uses the mini display port and therefore needs a USB-C to mini display port adapter. The difference between apple's Thunderbolt 27" display and their Cinema display LED 27" is very small, they are actually completely the same, except for the plug which is different. The Thunderbolt 27" screen has 2 connectors from the monitor for your Mac, a thunderbolt connector and a magsafe 1 connector for charging and the cinema display LED 27" screen has 3 connectors for a Mini display port connector, a USB connector and a Magsafe 1 charging connector. Both monitors also have a power cable. 

The adapter also works with all other Thunderbolt 1/2 or Thunderbolt 3 devices as it works both ways, so you can plug a thunderbolt 3 disc into your Macbook with Thunderbolt 1/2 or plug a thunderbolt 1/2 disc into your macbook pro with Thunderbolt 3. 

Does this work with your Apple screen? 

It may be hard to know, but there is a difference between Apple's screens, some of them have mini display port and others have thunderbolt. Fortunately, however, it is easy to tell the difference between them.

This adapter has a Thunderbolt connector, it does not work with a device with mini display port alone, in the same way it works with a thunderbolt hard drive, a mini dp adapter does not, it only works with the devices that use the mini display port.
Apple has created displays with both thunderbolt and mini display port, they've made 2 with mini display port, 24" and 27" and 1 with thunderbolt, 27", the difference is how many connectors you need to put in your Mac, all 3 monitors come with a magsafe connector for charging macbook's, they also all come with a video port that has the shape of mini display port/thunderbolt 1/2, but those with mini display port have a 3. connector sowm is a USB connector. 

So if your Apple screen has 3 connectors it is with mini display port, if it has 2 it's a thunderbolt screen. The mini display port screen needs a USB-C to mini display port adapter.