ATC is, put simply, a control loop involving Radar Screen->ATCO->Pilot->Aircraft.
Control loops can operate in two different ways: The controlling agent can explicitly command and require immediate (allowing for known system latencies) response. Alternatively, the controlling agent can give autonomy to its control objects, in which case it requires to be informed of all relevant changes by those objects through what are normally called "interrupts".
Putting that back into human terms. If the ATCo offers a "descend when ready", with the operational advantages that entails, then the courtesy of the "leaving FL for FL" interrupt seems like not too much to ask in return.