why we use PA,not other type amplifier?

Hi friends,
As we know, Ga,Gp,Gt are the gain of power.Then if we want to design two stages PA,normal use driving amplifier+PA,not 2 stages of driving amplifier.My qustion: if the input level is 0 dBm,design>24dBm output level
Why use driving amplifier(16dB)+PA(8dB),total is 24dB gain?Not use two stages of driving amplifier(12dB/stage)-----also gain 24dB?
Thanks,

Power amplifiers can only supply the wanted power with certain nonlinearity.Linear amplifiers will enter into saturation at high power levels..

Thanks for BigBoss'reply.You mean output level of driver amplifier will make the next amplifier saturation,and PA work in nolinearity.And if i want to design a total 18dBm output level,wehther i can design as: the first stage power gain =2dB,and the second stage power gain=16dBlet them all in linearity?