Why are prison sentences stacked upon each other?

Question

For example you’re reading about someone convicted of a crime and they get…2 consecutive life sentences.

Or where one guy gets 300 years and another gets 500 years?

I’m not sure of the additional years are meant as a punctuation to a sentence that reflects the crime, or isbthe judge/system trying to cover their bases in case of life extension becoming a thing? (And even if life extension is discovered, that person would be serving a sentence, and probably not eligible or rather able to get/receive whatever the extension would involve)

So, anyone wanna break this down for me?

*Not sure if my flair on this relates to economics, so putting it on other for now*

In: Other

Answer 1

if you get convicted of 2 crimes, and later one of the convictions is overturned on appeal, then you still have to serve the sentence on the other crime