If you make the fonts smaller, can you no longer read them because they are too small? Then you need a bigger monitor, not a higher resolution.

If you can no longer read them because they are too pixelated, you need a higher screen resolution.

On macOS: crisper text.

That’s on every operating system.

My understanding is that HiDPI mode on Windows requires each app to specifically support it, no?

On macOS too. On both operation systems 99% apps do though. Maybe its 99.9% on macOS vs 99.8% on Windows. But I'm using HiDPI on both and it was a long time ago that I encountered an app that didn't support it.