Reading this again after @amaclin’s answer, I realize that you might have indeed been asking about the input amounts rather than the output amounts. If so, you are correct. The input amounts are not explicitly specified in the transaction as they are specified by the prior transaction that created the UTXOs. A spending transaction only needs to specify which UTXOs it exactly spends, but the information about the output script and amount are already present in each node’s UTXO set.
If you were asking about output amounts, see my original answer below:
It depends on the type of transaction you are building.
If you are sweeping a wallet, consolidating into a single output or depositing several whole UTXOs to a service, you probably do not care about specifying the amount of the output. Rather, it is implicitly defined by the inputs and fees.
However, in my experience, most transactions are created to make one or multiple payments. When you make a payment transaction, you do want to send a specific amount to each receiver of the payment. Only the potential change output amount would then be determined by what remains from the inputs after deducting the payment amount and the fees.
