In early 2017 Adobe added Reference View, a new viewing mode, to the Develop module in what is now Lightroom Classic (Lightroom 6 users unfortunately miss out on this feature). If you have Lightroom Classic that means there are now three ways to view your images in the Develop module.
Loupe view. Shows a single image. The viewing mode you are most likely to use while developing photos (below).

Before and After view. Shows both the before and after versions of the photo you are working on in the Develop module (below).

Reference view. Shows the active image (the one you are developing) and a reference image together for comparison purposes (below).

Four Uses for Reference View in Lightroom
I can think of several uses for Reference view.
1. Consistent developing. Reference View helps you match the color treatment, White Balance and contrast of another image.
2. Emulating the look of a JPEG created in-camera with a Raw file. For example, the Acros black and white film simulation setting in the latest Fujifilm cameras is getting a lot of positive feedback. With Reference view you can match the Raw file to the JPEG, and create a Develop Preset so you can apply the settings to other photos.
3. Emulating the look of a photo created in a plugin. Open a photo created in a plugin as the Reference image, to see if you can create a similar look in Lightroom.
For example, here I have a Raw photo on the right, and the same photo processed in Alien Skin’s Exposure on the left (below).

After a few minutes work I came up with this. My Raw file doesn’t look exactly like the photo created in Exposure, but it’s close (below).

4. Emulating the processing used in a photo saved from the internet. You can use a photo you found on the internet as the Reference image, and see if you can create a similar look in Lightroom. This is a useful exercise if you’re wondering how a photographer created a certain stylized look in their images.
2 comments
Today I followed up on a note I put in my OneNote photography notebook with your name. Glad I did. There are a number of social/web based opportunities for learning. I often check in with Ted Forbes’ The Art of Photography, PhotoJoseph, and Anthony Morganti. I found your article on speeding up Lightroom Classic as I just purchased a new computer and expected better performance than previously (I could not remember my image preview settings).
Have you written anything about LR Addis’s? I have the Nik collection but I’m not sure what edits are best done with LR prior or after editing in one of the Nik modules, particularly the color efex or silver efex modules.
Hi David, do you mean Lightroom add-ons? As a general guide you should do as much editing in Lightroom as you can, and only export to a plugin such as those in the Nik Collection when you want to do something that Lightroom can’t. The plugin is the last step in the editing process. Hope that helps!