That's my understanding: Windows doesn't want the security problems of people allocating disk space and trying to read back garbage deleted files left behind by

That's my understanding: Windows doesn't want the security problems of people allocating disk space and trying to read back garbage deleted files left behind by other programs in the hopes of finding user secrets, so if you explicitly ask it to allocate a file of a certain size, it (slowly) fills it with 0s first. But also, Steam's allocation step seems an interesting relic of Windows 9X design patterns, smaller hard drives, worse cache/temp folder options, and (much) slower download speeds. It probably isn't necessary and they could maybe design something simpler and better from scratch today. (But it's probably a "not broke don't fix it" thing at this point.)