The rationale as I understand it, is to have a universal ap…

The rationale as I understand it, is to have a universal app to SPV wallet interface that will work without dependency of legacy indexers or full nodes.
This is because at 1M+ tps scale the old way of sender broadcasting to network and receiver scanning the chain to find the transaction will be extremely difficult if not impossible.
Note that the current desktop wallet now includes a legacy bridge so you can send and receive from existing wallets to onboard easier, but the headless wallet toolbox does not.