Does hard disk continue to run while sleeping? The question is pretty simple. I just wanted to know for sure but couldn't find an answer on google. 

Does the hard drive continue to "run" while the computer is asleep?

I'd like to know if it does and if that's the case why does it need to?
It will almost never run while the computer is sleeping. Sometimes when you put the computer to sleep, the disk will run for a short time while doing "housekeeping". There are also options in the Settings for waking from sleep (thus starting the hard drive) for network access.

Closing the lid of a macbook will try to get it to sleep, but it may not always go there. If it does successfully sleep, then your drive should be inactive.