There are a few reasons. One is that sometimes there are different crimes where the sentence can be or need to be either legally or at least symbolically stacked on top of one another. A proper accounting of the crimes and the sentencings lead to that kind of eventuality.

I remember seeing a guy get 100 years as part of a three strikes law sentence for rape. On top of that he got 2 five year sentences for failing to register as a sex offender. That's 110 years. The last two just seemed kind of like an afterthought, but it's just how the law is administered.

You can also argue that if something is found to be amiss with the first conviction and sentence, instead of simply letting the guy out the government keeps them in prison because of the other convictions and sentences.

And finally, I don't know enough about the criminal justice system to know if people who get life without parole really stay in prison for life without parole.