Why You Want to Hibernate Windows
I think the hibernate feature is such as under-utilized and unappreciated feature of Windows. I guess most people just "sleep" their laptops and tablets.
Some people like to "shut down" their computer or laptop at the end of the day so as to conserve battery usage, or electricity usage if plugged in. Whereas, other people may like to leave the laptop on and let it go into "sleep" mode on its own. This is especially true if they are in the middle of a complex task with several applications opened (such as editing a document with a spreadsheet and browser opened to a particular web page). Because a computer can "wake up" from sleep very fast, it is much more time efficient to touch the mouse or a key in the morning and have all the applications as they were last night.
Sleep mode does require a bit of power consumption as the state of the machine is held in RAM memory. As battery life improves and as tablet PC consume less power, this sleeping will become a more and more better choice. And starting with Windows 8, hibernate is not a visible option (unless you enabled it in the settings).
However, if you want to converse battery, use hibernate. It gives you a comprise solution that may be the best of both worlds. Sure it does not wake up as fast as sleep, but at least is uses no power.
What does Hibernate Mean?
When you hibernate your laptop, it completely powers down and does not use any more electricity than if you had shut it down. Hence you have the benefit of energy conservation.
However, before it powers down, it saves the state of your computer to its hard disk. That way when you power it back on in the morning, your desktop will be exactly as you had left it last night with all the same applications opened in the same state. No more having to launch all the applications again and opening up all your documents.
To bring your computer back up out of hibernation, you press the power button exactly as if you are turning on your computer after a shut down. In Windows Vista, you will get the message displaying "resuming windows" indicating that your computer is restoring the state that it has saved prior to hibernation.
Granted that it take a bit more time for the computer to come up from hibernation than from sleep. It will still come up faster than booting up your computer fresh after a shut down.
So hibernation gives you both energy conservation as well as the convenience of saving the desktop as you had left it.
What are the drawbacks of Hibernate
Software applications can have a special type of coding error called "memory leaks". These software bugs cause applications to use up RAM memory and not release that memory after it is done using it. Over time, your computer will have less and less available memory because applications with memory leaks are hogging the memory and not releasing it when it should be. Because of the lower available memory, your computer performance may eventually degrade.
It is too idealistic to expect all software applications to be free of such memory leak bugs. So the remedy to this solution is to shut down your computer and start it up again. By shutting down you computer, it frees up all RAM memory even if an application has memory leak bugs. So when you start up your computer from fresh after a shut down, it will have all the available memory it is supposed to.
Hibernation does not have this restorative effect on your laptop because it saves the memory state and restores the same memory state when it comes back up.
So it is still good to shut down your computer every once in a while.