Especially the second sentence is of importance. A watch-only wallet is nice for monitoring and has less risk in a potentially insecure environment. But you need to be sure that you still own the private keys which may reside in another wallet which is stored more securely. Otherwise you don't own aka have control over the coins in that watch-only wallet.
BTW, coins are always only "on the blockchain", wallets only store the private keys that enable you to "move coins on the blockchain".