Read-only mode or Watch-only wallet means you only can see your trasnaction history and balance. You create such wallet to follow up your wallet transaction, balance without needs to copy and paste your addresses to block explorers and check those details.
There is another useful use case of watch-only wallets. If they contain your master public key, which is the recommended way to create them and not mess around with individual public keys, you can also generate new receiving addresses for your wallet without having to power on or access your cold wallet.