An advantage of keying the buffer stage in a transmitter is that:

Keying a transmitter turns the RF output on and off to form Morse code. If you key the master oscillator directly, the sudden changes in load and voltage can cause the oscillator's frequency to shift slightly during the keying envelope. This unwanted frequency shifting is known as 'chirp'. By letting the oscillator run continuously and keying a subsequent buffer or amplifier stage instead, the oscillator is isolated from these sudden load changes. The buffer provides a constant impedance load to the oscillator, ensuring that its frequency remains stable. Keying a buffer does not eliminate key clicks (which are instead managed by an RC filter to shape the waveform) nor does it inherently eliminate high RF voltages. Its primary and most important benefit is frequency stability.