Another reason you should never discard you seed or private keys even if you don’t use them/see a need for it again.. you never can predict if/when the need for it could arise.
It can be used to prove ownership or identity by signing a Bitcoin message from private key of your known and used address, but it is not the only case to think of.
Someone can send you bitcoin to your past addresses that you no longer plan to use it in future, however it does not cost you anything to keep storage and backup of your past wallets and addresses.
Someone can send you bitcoin without asking you, and it can be a fortune. It raises another concern, that if it is a mistaken sending, will you keep it or return it to the sender in such case, it depends on you.