Do I have to inform the hotel about my lateness for check in? [duplicate]

It's my first time to personally book a room, and I'm a little bit confused with the notion of "check in".

Let's say I am supposed to arrive at the hotel on Aug. 15th @ 01:00 am (the room is fully paid for all of planned night) whose check in time is 03:00 pm. Then, if I doesn't book Aug. 14th, I have to wait 14 hours till the check in time (!); however, if I book Aug. 14th, as well, then I can directly get to the hotel at 01:00 am.

But I do not know whether I must inform the hotel about this plan or not, because some people say if you don't be at the hotel some time around the check in time (say mostly one hour later), they will maybe give your room to another one.

Is it true?! I mean, I have already paid the price for Aug. 14th; why do I have to inform the hotel about my lateness?