Stating the three reasons

The required three reasons are :

(i) A reasonable length of the transmission antenna.

(ii) Increase in effective power radiated by the antenna. 

(iii) Reduction in the possibility of a 'mix-up' of different signals 

Detailed Answer :

For a long distance Transmission of message signal we need modulation because of following reasons.

(i) Low energy: The message signals when converted to e.m. waves, do not have the sufficient energy to travel up to long distance ,because of their low frequency. Hence these message signals are modulated with high frequency carrier signals before being send because carrier signals have high energy for long distance transmission.

(ii) Size of Antenna: For the effective Transmission by an antenna ,the size of antenna should be at least of the size , λ/4 where l is the wavelength of signal to be send. Thus for an em wave of audio signal of frequency is 20 kHz, we need an antenna of size 4λ/4 i.e. nearly 3.75 km high, which is practically impossible. Hence, these low frequency signals first modulated to high frequency signals before transmission to get the proper size of antenna.

(iii) Mixing of signals :- When number of signals are transmitted simultaneously, all these signals will get mixed up and at the end we get mixed signal, which is very difficult to separate. Therefore to remove this limitation, transmission is done at high frequency and a band of frequency is allotted to each user at the end , as done for radio and TV channels.