Ah! I get it. Some If the program have 100 "cells" of memory, the first time it get blank, the second what was there before?

Clear the data is costly in the long run? I have read yesterday about data oriented design and it make me understand that now is costly to access memory.

That's exactly it. If you're going to dump data into that memory that is guaranteed to overwrite whatever is already there, then there's no point in wasting cycles zeroing it first.