The fan switch will still activate the fan if you keep it wired in so it's not completely redundant.  Say if you've wired it to trigger off the light but you're showering during the day then you can flick that switch to turn the fan on without using the light.

If you are triggering off the light switch instead of the fan switch and showering with the light off you can turn on the lights to pretty up your face or as you brush your teeth to trigger the timer on your way out.