Since gas-based hot water heaters burn natural gas or propane, they require venting to keep potentially dangerous gases out of the building. In general, this gas venting is automatic and requires no extra power, but units that require power venting are sometimes used in especially large systems.
Fortunately, newer gas combustion processes are far more efficient than those used in the past. As a result, there's generally little carbon monoxide generated when they burn. Despite this, venting is still recommended for gas water heaters since they burn so much natural gas. Even a slight inefficiency in the combustion process can cause carbon monoxide levels to rise significantly in a short period of time.