One of IOTAs main use cases is to provide a way for M2M (machine to machine) micro payments. Therefore services can be called by sending a transaction, including payload + funds, to a specific address.
Since addresses should not be reused, they have to be changed after every withdrawal. As also discussed in A receive address is made public for donations, how do I safely withdraw funds? the consensus is to change the receive address before withdrawing.
How can this be accomplished in an automatic matter for M2M transactions? How can a machine request the current address to issue the request to? The service's address may have changed due to a withdraw.
Consider a simple SMS-service that receives a transaction, including number, message, value, and sends an SMS accordingly. How can another webservice request the current address (to which to send the transaction to) from the SMS-service?