It seems nonsensical: Why sentence a defendant to more than one "life" sentence? Life is, after all, life. But there are actually practical reasons why courts impose sentences that exceed "life."
In some jurisdictions, a "life" sentence is a misnomer in that it can come with the possibility of parole. Depending on the state's law, a defendant may be eligible for parole after a set number of years, like 20, 25, or 40. A defendant who has served the minimum sentence can apply to a parole board for release. (A judge typically hands out the initial sentence but takes no part in the release decision.)
A defendant who receives life without parole cannot apply for release. The sentence commits the defendant to a life behind bars (except in rare instances, as where the person receives some kind of clemency).
As you can see, certain life sentences don't always result in actual life imprisonment. But even where the sentence is life without the possibility of parole, consecutive (back-to-back) life sentences may serve a practical purpose. Most often, multiple life sentences arise in murder cases involving multiple victims.
To take a situation involving the possibility of parole, suppose that a defendant is on trial for two murders. The jury convicts him of both, and the judge sentences him to consecutive life sentences with the possibility of parole. State law allows the defendant to apply for parole after 20 years. By sentencing the defendant to consecutive life sentences, the judge has likely ensured that he will be behind bars for at least 40 years.
Now let's say the defendant received consecutive life sentences without the possibility of parole. Here, the defendant appeals the convictions and a court overturns one because the prosecution didn't comply with its discovery obligations. That takes one life sentence off the books—at least until the defendant can be retried. But the other life sentence remains in effect.