The Viewer can display more than just a single image. The Viewer modes are Multi, Primary, Three Up, Compare and Stack. These are accessed through the View menu or via a pop-up button on the bottom of the main window.

The first two look like this. I have increased the area devoted to the thumbnails:

Primary only displays the primary selection (thick white outline). Multi will show all images selected (including those with the thin white outline). Switching to Three-Up does just that. Three images are always displayed. As you click on thumbnails, the selected image is put in the middle and the adjacent ones to each side:

Zooming still works of course. All the images are zoomed 100%:

Compare mode puts an orange bar around the selected image and locks it on the left:

Now clicking other images changes the right hand image only (white selection box):

Turn off zoom and set the Viewer mode to Stack. Click on an image in a sack and hit return. The image is promoted to the pick of the stack and the Viewer shows it on the left plus whichever image you select to compare with it on the right. Like the rightmost better as a pick, just hit return and it is promoted t the pcik and displayed on the left:

The Three-Up mode can be used for comparison too. Select an image and hit return to lock it on the left:

Turn off the Projects pane with W and use shift-W to rotate the display: