There are a couple of benefits. First of all, it speeds up the cookie baking process because you don’t have to wait for butter to soften.  That allows you to be able to bake cookies anytime because you don’t have to plan ahead. The cold butter also keeps the cookie dough chilled, so it helps eliminate that step from the baking process.  Chilling the dough is great when you want a thick cookie.  Using cold butter gives you that awesome thick and gooey texture without refrigerating the dough.