reason modulation
In fact, there are many misconceptions about modulation:
1- using high frequency doesn't improve the signal in means of the the quantity of noise incurred on the signal because white noise ( the type of noise dominant in our systems) exists by the same anount in all the frequency spectrum.
2- imagine yourself in a class, and the teacher asks a question, now if everyone of the bright students talk at the same time, no one will understand a thing, something called interference....but if everyone of them is given a time slot to talk into, so that everyone will understand. the same thing happens when you use sinusoids, you must use different sinusoids in order to accomodate different user, but how can this be, if our voice signals (all human beings) have approximately the same bandwidth? ....
This can be accomplished by transating this signal into a higher frequency ( and not giving this spectrum to anyone else as long as this user is using it)
3- translation of this information signal into higher frequencies enables us to put this information into many forms, and because we are using sinusoids as the means of doing that then it means that we are restricted to what this signal offers us ( which is limited to amplitude, phase and frequency) so that we can convey this information via manipulation of one or more of these three.
4- another misconception is the idea that low frequency signals get attenuated more than high frequency signals, which is of course WRONG, because it's the reverse thing actually, but as was mentioned before, it's the antenna hight that makes us use the high frequency spectrum, since we need an antenna that's relatively in the order of the wavelength to obtain a good gain....
i hope i didn't bore you