Why can you get multiple life sentences?

I get recommended true crime Videos from time to time, and what stands out to me is the sentencing. I girl (underage) murdered her mother and got life in prison plus 10 years for tampering with evidence. But whats the point of adding another 10 years when she's already gonna rot in prison for life? I talked to a friend about it, and they said a life sentence means a 20 year sentence, not literally a life sentence. But she was given the choice to settle out of court, which would mean she goes to prison for about 25 years without parole and gets out at 40. The other option is her going to court, proving that she's mentally ill and won't get jailed or she gets a life sentence + a Charge for attempted murder of father and one for tampering with evidence. But if a life sentence is 20-25 years, why use two different words if they are the same thing? The same goes for giving someone several life sentences. If a serial killer gets 300 years in prison and has 25 years added to it for another crime, whats the point of it? The result is the same regardless. My friend said, that it was meant to symbolize the severity of the persons crimes, so that they dont possibly get released. But why would anyone free a serial killer, even if he got only 80 years. That makes no sense to me