If you want to invest your funds and coins, it is also a MUST to invest in a wallet that will secure it. Yes, you can use a written program with a custom key to keep your funds but there is no assurance that it is truly secured and be compatible with computer software and programs in the future. Take note that this is your fund and you are the one accountable for its probable loss. So whether you invest in expensive but secured hardware wallet, use custodian wallets, or rescript programs, just make always ensure the security of your own funds.