A great culture is the pinnacle of great leadership. If you've done your job right it means that people are doing great things even when you're not there to guide them.
Creating a great culture isn't absolutely necessary but it will make your job as a leader much easier, well eventually anyway, it's definitely not easy to create a great culture. Culture is how people behave when you're not there. You're in for a rough time if you need to be around leading people all the time.