Amplification means increasing the amplitude of waveform . Like we convert sin(theta) to 5sin(theta) then amplitude becomes 5 times. Modulation involves changing the amplitude/frequency/phase of carrier wave according to message signal. Modulation is necessary for transmission of signal as original signal cannot be transmitted to long distances (due to more signal loss and height of antenna required is large) so modulation decreases the height of antenna required . However inspitee of modulation signal becomes so its amplitude is increased by amplification.

However inspite of modulation signal becomes weak while propagating through the medium hence its amlitude is increased by amplification.