Smart Previews to the Rescue
Lightroom 5 introduced Smart Previews and this is what Adobe has to say about them:
"Easily work with images without bringing your entire library with you. Just generate smaller stand-in files of your full-size images. Any adjustments or metadata additions you make to these files will automatically be applied to the originals."
NOTE: Lightroom 6 introduced the option for the processing engine to "prefer" Smart Previews during editing. However, our testing has continued to show that taking the originals offline during editing is still substantially faster than taking advantage of this new option.
When you create Smart Previews you're telling Lightroom that you want to be able to continue working on your images even though the hi-res originals aren't available. When you tell Lightroom to create Smart Previews, it builds smaller versions of the original images and stores them inside the catalog. If the original images become unavailable at some point, Lightroom just keeps chugging along and you can continue your edit using the Smart Previews.
There are two use cases I can think of immediately: you store images on external drives that don't always have to be available; you want to be able to deliver catalogs to post-production houses or offsite editing staff without having to deliver massive originals. But all those reasons completely miss the best reason . . .
SPEED! Lightroom FLIES when editing with Smart Previews because the files are so small!
To be fair, you have to coax the speed out of Lightroom but it is super easy to do. The trick is to make Lightroom think the original files are offline. This is how it works:
The only minor downside (in our estimation) is that you can't zooom to 100% with Smart Previews. You can still zoom in enough to check focus, but it's not nearly 100%.
Hopefully Adobe will add a setting that causes Lightroom to "prefer" the Smart Previews for normal editing and only use the original for 100% zooms, etc.