It serves no purpose in preventing future crimes. First, because you've already caught the criminal, the proposed alternate sentence of life imprisonment without chance for parole already handles preventing that individual from committing further offenses. And second, it's been proven repeatedly that ratcheting up punishment as some sort of deterrent doesn't work because most people committing crimes either believe they're going to get away with it or aren't in a frame of mind to care about any penalties (e.g., crimes of passion).

It also fails to be an efficient way of dealing with a caught criminal. The combination of incarceration time, resources spent on appeals, and the cost of the actual execution methods makes death row inmates more expensive than lifers. We actually pay more to put people to death than we would jailing them for the rest of their lives.

Instead, people fight so hard to keep capital punishment around because it scratches the itch for vengeance.