elementary question
for acting as voltage amplifiers , to take the majour part of the input voltage and deliver the most part of the output voltage
consider a voltage divider , the voltage signal is mostly delivered to the high resistance , so if u deliver voltage u need to have a very small output resistance to give any small load most of the voltage signal , on the other hand if u accept voltage signal u have to have a large input resistance to accept most of the sources voltage (i.e. lower drop on the source resistance)