Background app refresh permits an app to refresh its contents/data when it is not currently in use. How much turning this off for specific apps actually improved battery performance depends on the app. Turning it off does not make apps unusable, it just means a slight delay when to switch to one in updating its data.
About Multitasking on your iPhone, iPad, and iPod touch - Apple Support
After you switch to a different app, some apps run for a short period of time before they're set to a suspended state. Apps that are in a suspended state aren’t actively in use, open, or taking up system resources. With Background App Refresh, suspended apps can check for updates and new content.
If you want suspended apps to check for new content, go to Settings > General > Background App Refresh and turn on Background App Refresh. If you quit an app from the app switcher, it might not be able to run or check for new content before you open it again.
You might test it by turning off WhatsApp and seeing if you notice a difference. You can always turn it back on.