Why should I use a spindle rather than a router?

The spindle is quieter and has more consistent torque (It doesn't get "bogged down" when it starts cutting into material). It can be used with collets of varying sizes (not restricted to imperial sizes). It is water cooled and so it does not blow air downward to cool the spindle like the router does. The spindle is also machined with much higher precision than typical routers and will have a much longer life.