While I know that many highly successful people operated quite well at even fewer than 5 hours of sleep a night, I believe that our bodies are programmed to sleep for at least 1/3 of our wakeful portion of our life cycle. Now, I know that there's research out there to support some of what I'm about to say and that some of what I believe about sleep is based on my ideas only; however, this post isn't about the evidence, just my beliefs.
We need sleep to repair our bodies. Depending on how active we are, a good portion of our waking hours involves activities that stretch, tear, or deplete our bodies' resources. Sleep puts us (well, most of us) in an inactive state that allows those wounds, however small, to heal, strengthening us for the next day. I also believe that too much sleep sees those healing processes continue to work, but actually in ways that promote inactivity. Our bodies need the activity to balance the healing.
We need sleep to "save" our memories. Not only does sleep help repair and replace our physical selves, it does the same to our mental health. I believe that it is in sleep that our brain's mental pathways are reinforced and interconnected. This is where dreams come from. People who remember their dreams tend to be people whose sleep has been interrupted. Waking from a deep sleep cycle catches a glimpse of the process that our brains use to connect our memories from the day to archived memories and thoughts. This process is what allows us to wake up able to complete a task that had been nearly impossible the day before. A lack of sleep, thus equates to a lack of memory and creativity as well.
Our bodies are amazing, self-repairing machines. Getting enough sleep (the amount varies from person to person) is essential for our physical and mental well-being.