What is the Need for modulation
Modulation is, generically, a means of shifting information from one frequency domain into (typically) a higher frequency domain.  This provides a number of benefits; among them:


Two or more input signals with matching or overlapping frequency domains may have their information shifted into disjoint frequency domains; if such signals travel together through a medium, having their frequency domains be disjoint will allow them to be separated.
In many cases it's easier to uniformly handle signals where the minimum frequency is a substantial fraction of the maximum, than signals where the maximum is many times the minimum.  An audio signal whose frequency content is 20-20,000Hz has a 1,000:1 spread between the minimum and maximum frequencies.  If such a signal were amplitude-modulated at 1MHz, the spread would be about 4% [from 980,000Hz to 1,020,000Hz].  Even if there were no other radio communications anywhere in the world, trying to design an antenna which could work well with a 1,000:1 frequency spread (from 20Hz to 20KHz) would be very difficult.  Designing an antenna to deal with a 4% spread would be much easier.


Some use cases for modulation exploit both benefits; others only rely upon one or the other.