Sanctions are an important symbolic message to Cuba. America will not allow Cuba to benefit from free trade if it continues to oppress its people with an outrageous Communist system. Putting pressure on the Castro family through sanctions is the best way to force them to change their ways and reform.
Sanctions are not simply done for economic reasons. Sanctions let a country know that their past or current actions are unacceptable to the world. Without sanctions, a country would assume that what they've done or currently doing can be continued without any penalty from other countries, even if it is against international law. The economic consequences of sanctions are not the only thing that's important,