Taken from the Tartybikes website:
Double Butted spokes build more durable wheels than cheaper plain gauge. Why? The spokes can flex in the centre, rather than transferring the load to the head, which is where spokes are more prone to breaking.
It isnt required, it is possible to use a hammer and a punch but you have to be carefull. Also tarty can take apart your hub and change the shell for £10.