That actually has a function. First is as a life sentence usually isn't actually until you die, but you sit x years (usually 20-30) and then can apply for parole and might get it. Multiple sentences can be set to run one after the other, increasing that time - though usually in that case you just get "life without parole".
Another is if you have committed multiple crimes. Even if later they overturn one of them because of new evidence etc, you still have the others left keeping you in prison, instead of having to bring it back to court to figure out if that part was or wasn't enough to give you a life sentence instead of a shorter one.
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