Sky vent startup vent valves bypass steam around the turbine to ensure the unit does not come up to temperature too quickly.
An issue with bypassing steam through the turbine bypass valve during initial plant startup is that it causes a major shock to the the valve and piping systems. Therefore, steam is typically bypassed directly to atmosphere through the sky vent valve in order to warm the turbine bypass valve prior to opening.
The large pressure differential across sky vent valves at initial plant startup can cause high noise levels and mechanical vibration that needs to be attenuated. Once the valves are closed, they must maintain tight shutoff for long durations because any lost steam yields less efficiency and less power production.