aftertouch give you the ability to add expression to your playing , basically if you press harder on the keys after the initial press you can program the synth to vary patch parameters.
For example you can take a violin sound and use the after touch to apply vibrato/tremolo effects to the sound just like a violin has when you wiggle your fingers on the strings ( im not a violin player so excuse the amature expression).
I use it on pad sounds to control filter sweeps , the harder i press the higher the filter cutoff goes ( or lower depending on wether its a high or low pass filter )
Once you get used to using it you won't like playing without it.