When you own multiple Macs that are linked to the same iCloud credentials and you also have iCloud Photos turned on, you may want to not sync the iCloud Photos library to every one of your Macs. Is that possible? Yes, but you need to read the fine print before checking or unchecking boxes.
Apple uses iCloud Photos (the newish name for iCloud Photos Library) as its keystone for letting you have your photos and videos on all your iCloud-linked devices. Through a combination of sync of thumbnails and full-resolution media, you can access all your assets wherever you are, even when you’re off the internet.
However, iCloud Photos requires a certain level of commitment. Any device linked to iCloud Photos has to retain the full library—even if each item of media is just a thumbnail—and you can’t pick and choose what’s synced.