The advantage of the autorebuild is if you are not around to initiate it, it will happen automatically. But, if I recall correctly, it only happens on a restart anyway.
As mentioned, it is usually better to set it to manual so that you can see what is going on before starting the rebuild. Often it works if the errors are minor, but more than likely at some point the drive will have to be replaced.
Not a big deal one way or the other as you can always set it back to manual and reboot the NAS if you need to.