Those functions explicitly? I can't find any definitive explanation on why they exist.

It looks like nowdays ZeroMemory() and RtlZeroMemory() are just macros for memset().

Here's an article on some of the RECT helper functions. Relevant for the 8088 CPU but probably not so much today: https://devblogs.microsoft.com/oldnewthing/20200224-00/?p=10...