A hardware wallet is an electric device that has a limited life span (someday will stop working), so for your coins' safety you need to keep the wallet seed safe and offline/physical. Depending on people's preferences some save it on steel plate. Yes, it's fire proof, but if you need somebody else to engrave it you have already a problem (you should not give the seed to anyone, you know that, right?) and a seed plate stands out more than a piece of paper. On the other hand, many just cannot be careful enough with just a piece of paper.
Everything has its lifespan even metal plates that can be destroyed in catastrophes so having proper backup methods is very important and it is vital to have multiple backups rather than only one backup. As said, in case one backup is damaged, you will be able access other backups that are usable for your wallet recovery.
This advice is applicable for all backup methods, not only for hardware wallet and seed phrase backup.
How to backup a seed phrase.