It seems silly for a judge to sentence someone to over 100 years in prison but the reason that a judge does sentence someone to over 100 years in prison especially if they have been sentenced to life in prison is to ensure that the inmate never gets released from prison.
Basically someone has done something so serious that means that the person would be a threat to society if they were to be released from prison.
With a life sentence there is always a chance that the inmate could be released on parole or through appeals.
With a sentence of over 100 years it means that the person will not be up for parole until those years passed by and so by saying the prisoner needs to be in prison for over 100 years.
Most people will not even live to 100 years so that ensures that the inmate will be dying in prison.