You can store and transact coins and chains that are not natively supported on ZERO.
ZERO will be able to show the most common chain and coin details.
However, in some cases the transaction can't be decoded or part of the decoded transaction can't be interpreted. Make sure you understand the risks of blind signing and take appropriate measures to protect your funds (see How to stay safe on web3?).
The obvious advantage of using Rabby Wallet paired to ZERO, compared to simply using Rabby, is that the private keys are protected by ZERO.
You can simply add any network that is supported in Rabby, create transactions and sign them with your ZERO. Like you would do for a coin or network that is supported on ZERO.
All logic to create transactions is in Rabby. The ZERO is 'only' needed to sign.