You might need the serial number when you register your Apple TV or request support.
Find the serial number of your AirPods - Apple Support (IN)
Identify your iPhone model - Apple Support (IN)
Identify your MacBook Pro model - Apple Support
How to Check APPLE TV 4K Serial Number - Where to Find Apple TV Identification Number - Video Guide
If you're asked to confirm your Apple ID when you contact Apple – Apple Support (UK)
Identify your Apple TV model - Apple Support
Get system information about your Mac - Apple Support (IN)
How to find the serial number or IMEI for your Apple Watch - Apple Support (IN)
View your purchase history for the App Store, iTunes Store and other Apple media services - Apple Support (IN)
Find the serial number of your Apple display - Apple Support (IN)
How to Check When Your iPhone Was Manufactured - AppleToolBox
Official Apple Support
Apple Planning Switch to Randomized Serial Numbers for Future Products Starting in 'Early 2021' - MacRumors
How to Chat with Apple Support Directly From Your iPhone