That's a side effect, but it's more about being able to share control over the funds with the other party without fully committing them.
Then they can let you instantly spend them on things.
There is substantial complexity, and they would have to monitor all the addresses used to make sure that you don't try to commit an early transaction to the blockchain and bounce the funds out of the shared address.
Anyone offering blockchain services should anyway have automatic processes making sure that the blockchain records of their active addresses correspond to their internal accounting, but it is repeatedly the case that these operations don't even do internal accounting.
Then they can let you instantly spend them on things.
There is substantial complexity, and they would have to monitor all the addresses used to make sure that you don't try to commit an early transaction to the blockchain and bounce the funds out of the shared address.
Anyone offering blockchain services should anyway have automatic processes making sure that the blockchain records of their active addresses correspond to their internal accounting, but it is repeatedly the case that these operations don't even do internal accounting.