They don't really hold rooms back in the sense you are referring to. They offer available inventory. It doesn't really serve any purpose to just hold rooms back for no reason
Sometimes blocks can be held for something like a wedding or other event, not just individual guests have them booked
A matter of rechecking to see if someone cancels or if blocks are returned to inventory and available for booking
So yes, bottom line is, availability opens up all the time. I check multiple times a day if I am after a specific resort especially during a specific promotion.