Some people use the "hold" feature while they run the figures past spouse/family/travel party. Or maybe to give them time to make sure they can get the time off; though at this point the deposit is refundable (other than concierge).
I think most of the "holds" are done by TA's, who contacted
DCL on behalf of a client and then need that client's sign-off to the specific fare quote. Some will drop; others will be confirmed with a deposit.
A "placeholder" is not the same as a "hold." A placeholder is an Onboard Booking (OBB) that someone makes while on a cruise; sort of a certificate for a discount on a future cruise. Yes, placeholders -- and any reservation -- can be changed to a different stateroom or even a different cruise date. Placeholders must be used within 24 months of when the OBB deposit was made (during the prior cruise).