Pretty much anything related to the work environment. This can include maintaining a safe work environment, accommodations for specific employee needs, handling employee disputes.

That's a fraction of what HR does. Compensation, benefits, hiring, wellness, training, performance reviews, etc, etc. I doubt that even 10% of what HR does is tightly related to a physical space.

I don't see how this isn't still necessary.

Do you see how these problems greatly diminish if people cannot interact physically?

You don't see why maintaining a work environment wouldn't be necessary if the entire company were allowed to work from home?

The work environment remains, it just shifts location.