That is it precisely in simple terms, and it is not so difficult to explain. ARDR is the main chain token. The main chain secures and validates the transactions in the whole platform. This means any blockchain within Ardor that wants its transactions to be registered so anyone within the Ardor platform will count them, needs to anchor them in the main chain, which requires paying fees in ARDR. Blocks including validation for all transactions done in all the blockchains in the Ardor platform are registered, and the corresponding fees paid, in the main chain. So ARDR is the token that can be sent to and from the main chain and all the child chains, confirming and securing all transactions and contracts made in the whole network. Your question can also be applied to other platforms, say about Ether - so, besides being the main network token, what else do you do with it?