Re: communication
well there are a couple of reasons, one of which has been mentioned by DarkJedi. baseband signals dont have enough energy for propagation. and for low frequencies you will have to use very big (BIG!!!) antennas. try it yourself;
C = f * (lambda)
calculate lambda for different values of f and divide it by 10 (optimum). thats the minimum value for your antenna.
in addition to these reasons there is the problem of bandwidth usage. if there is communication being done in the baseband then, all of the bandwidth will be wasted. but with modulating the information onto a carrier we can use the concept of frequency division multiplexing for efficient bandwidth usage.
then comes the issue of SNR (signal to noise ratio). a high frequency carrier will result in low SNR. consider that you are communicating on baseband (low frequencies) then all the nearby noise generators (fan, generator, high tension wires, your car) will interfere with the signal. but modulating your information on a high frequency carrier will make it noise immune (to an extent, that is).
get a good book related to communications like the one by Lathi or Haykin
i hope that helps