Notorious Boston gangster James "Whitey" Bulger was sentenced to two life terms, plus five years, in prison on Thursday for his conviction on 31 counts of weapons charges, money laundering, and racketeering. Adding those five years probably seems a bit pointless for someone who will die in prison, but there is a logic behind it. So where did that number come from, and why tack it on to an already unneeded second life sentence?

The source of that half-decade addition comes from his conviction for Possession of Firearms in Furtherance of Violent Crime, which has a mandatory five-year minimum. In fact, five of the 31 criminal counts that Bulger was charged with were based on illegal use of guns, including possession and transfer of unregistered, serial number-less weapons. Some of those machine gun charges resulted in one of the two life sentences, while the second life sentence was based on two racketeering charges.

But why bother to add them on separately? Obviously, Bulger cannot spend more than his own one life in prison, which at 84-years along, may not last too much longer. On its face, it might appear like a judge showing off or flexing power, but the reason is a thoroughly sensible one. Each charge is considered independent of another during the trial (as Slate explained back in 2005), and so each must be considered separately during sentencing.

More importantly, multiple sentences matter a great deal during the appeals process. Should Bulger choose to appeal his conviction — he has a 14-day period to do so — a reversal on just one charge would not wipe out his whole punishment. By adding two life sentences, plus another five years, he would need many more successful appeals to avoid prison. As Lehigh Valley Judge Robert Steinberg explained the idea to The Morning Call back in 2011, "The judge isn't doing it as a joke or to be facetious … The judge wants to make sure the person never gets out." And at Bulger's advanced age, five years alone might be enough to accomplish that.