When it was designed there was no UTF-8. You had to know the code point; and file systems like FAT did not encode the code point, so file names would be interpreted by whatever the current code point was.

So having a file system and other APIs support Unicode natively made a lot of sense at the time; it simplifies many things including deploying into organizations that have global users. This is now over 30 years ago. And the A vs the W suffixed APIs were not simple ascii; they were multi-byte, so to work with strings you still had to know the code page. Obviously UTF-8 is much simpler for that now, but that wasn’t the world when this was designed.