The honest answer is you probably don't need one but the reason they are used is to provide isolation between the source of the signal and the load placed upon it. The buffer would have a high input impedance so it doesn't load the source and a low output impedance so it can provide more power to the load. For example, and it depends on the oscillator you are using, it may shift frequency if you place components on the output pin or the output voltage may drop as you draw current from it. The prevent that happening, you use a voltage follower, it doesn't amplify the voltage, you get the same out as you put in, but it prevents changes at it's output from influencing the source feeding it.
Brian.