What is the difference between twrp-[version]-[phone].img and twrp-installer-[version]-[phone].zip? I know I can flash the image and boot it. But what are the zips used for? I'm asking this because I noticed that images are often accompanied/listed with corresponding zip files - not just for TWRP. And I'd like to understand how they relate and how those zips are applied and when. 

For example: https://dl.twrp.me/enchilada/
The answer becomes clear once you read the install instructions:

You will need both files to install, as your device (OnePlus 6 aka enchilada) is an A/B device and thus has no recovery partition (recovery is part of the boot image there) – which means you cannot simply flash the recovery image using fastboot. So you download (or push) the ZIP to your device, then temporarily boot TWRP (fastboot boot *.img) and from there flash the ZIP.