I know that on a 64-bit version of Windows the "Program Files" folder is for 64-bit programs and the "Program Files (x86)" folder is for 32-bit programs, but why is this even necessary?

By "necessary", I don't mean "why could Microsoft not have made any other design decisions?" because of course they could have. Rather, I mean, "why, given the current design of 64-bit Windows, must 32-bit programs have a separate top-level folder from 64-bit programs?"

There are plenty of questions on Super User and elsewhere that assert "one is for 32-bit programs, one is for 64-bit programs", but none that I can find give the reason. From my experience, it doesn't seem to matter whether a 32-bit program is installed in the correct place or not.

Does Windows somehow present itself differently to a program running out of "Program Files (x86)"? Is there a description that shows exactly what's different for a program installed in "Program Files (x86)" instead of "Program Files"? I think it's unlikely that Microsoft would introduce a new folder without a legitimate technical reason.