I have transferred numerous photos from Finder on my MacBook Pro to the iCloud Drive. I have purchased 2 TB of storage in iCloud. I now see in 'About This Mac-Storage' that the iCloud Drive takes up 439 GB on my MacBook Pro's HD. I thought that I was transferring the photo files to the Cloud. Why do these files take up disk space on my MacBook Pro?
MacBook Pro Retina (2015 and later)
iCloud Drive FAQ - https://support.apple.com/HT201104 - iCloud Drive lets you securely access all of your documents from your iPhone, iPad, iPod touch, Mac, and Windows PC.
Rationale behind iCloud storage - Jan 2016 - https://discussions.apple.com/message/29713224#29713224 - "iCloud is not archival storage. It is not designed to be place to store your content off-device. That's what Dropbox or LiveDrive does better.
It is designed to easily sync your content between multiple devices. Photos, contacts, calendars, music and so on is accessible from whatever device you want in near real time. This is possible because iCloud is the place content is sent to and received from. An intermediate step. Not a destination, but a step along the way."
iCloud Deconstructed - https://discussions.apple.com/docs/DOC-12002
If you need extra storage for your files buy a hard drive.
Thanks Limnos for all of the information. It seems that I was trying to use iCloud drive for something for which it was not intended.
However, I did find the following information on an Apple support page. I suppose that what Apple considers 'recent files vs older files' will determine whether I can use iCloud Drive as a backup hard drive.
Store your files in iCloud and save space on your device
The files that you keep in iCloud Drive use your iCloud storage. And as long as you have enough space in iCloud and on your device, you can store as many files as you like.
If you need more space on your device, iCloud Drive can help. On your Mac, go to Apple menu 
> System Preferences > iCloud. Next to iCloud Drive, click Options and turn on Optimize Mac Storage. Then your Mac keeps all of your recent files on your computer, but keeps your older ones only in iCloud, ready for you to download when you need them again. If you want to free up more space on your Mac, follow these steps. On your iPhone, iPad, or iPod touch, you can search and browse files from all of your devices and download them only as you need them.
iCloud Taking Up Space on my MacBook Pro's HD
