In order to transact on blockchains, such as to supply funds to the Compound liquidity pool, you need to sign blockchain transactions using your unique key stored on your mobile device. That is why you need to have a self-custody wallet. Only you have access to your funds and only you can sign blockchain transactions from your mobile device. Self-custody wallets are growing in popularity, especially among twenty-somethings.
You can restore access to your funds using your unique phrase onto most wallets that support Ethereum-based digital assets.