Double or triple life sentences matter in most jurisdictions. First, "life sentence" in many places means something like "life sentence, or 20 years with good behavior or parole", so a double life sentence makes release less likely. Second, multiple life sentences usually come from different instances of the offense, so it reduces the chance that a single appeal or reversal will release the convict (for example, someone who gets 4 life sentences for killing four people and then it's discovered that one of the victims actually died of something else, the convict would still have 3 life sentences to serve).