Buffer amplifiers are used to avoid " frequency offset " phenomena in such a tranceivers. Because , input impedance of an power amplifier is changed by signal amplitude and this may be reason of pulling effect at the output of VCO ( or simply oscillator ). When impedance is changed by the signal , oscillator frequency is also changed.
To provide sufficient isolation between power amplifer and Oscillator ( or VCO) , a ' buffer amplifier" is put, hence the impedance that is seen by oscillator rests constant and pulling effect is minimized.
This is true also mixers ( especially diode mixers ) . When you drive into mixer from a VCO , you should use a buffer amplifier to avoid "pulling effect"
Rgrds