if you have roads shared with pedestrians and cars (and bikes!) you can build denser cities.

I lived real downtown in Tokyo and my street was like "1.5" lanes wide (if cars were coming in both directions one basically needs to pull over and stop). I could just walk in the middle of the street. There was no sidewalk. No street parking of course. Cars would drive down at 15km/h or whatever, and slow to a crawl if people were in the street.

Straight lines are efficient walking routes, and ... well... that might involve just crossing the street directly! Every layer of grade separation gets in the way of that.

End result of all of this is less pavement to maintain, slower drivers (-> safer!), good walking and cycling conditions, etc etc etc.