I don't think putting it to sleep instead of shutting it down preserves much energy. Well, booting and shutting down involves quite a bit of harddrive, but it depends on how long it'll be 'off' or 'asleep', really.
Why you _would_ put it to sleep instead of shutting it down, however, is a practical reason: It's much faster to wake up than to boot. In fact, it takes about zero seconds to wake-up from sleep, while it takes between 30 seconds and two minutes to boot the machine (depending on login items etc.) or five minutes until you've reopened your projects and working environments etc.