[samtheman]

While the criminal sitting in prison, he's not committing new crimes - to you or your neighbors. That's one way you're protected. There are other ways too..

There's deterrence. Imprisonment and threat of imprisonment deters crime.

[The_Reader_David]

Recidivism rates and the attitude among some criminals that prisons are 'schools' that make them into "better" criminals when they come out suggest that the deterrent effect is oversold.

The thought experiment in Peter Moskos's In Defense of Flogging bears some consideration.