We don't really have dirt here, our "soil" is mostly sand which works out well for keeping our run clean (I can easily rake it). Maybe plain dirt is harder to keep clean? And it could get muddy when it rains. I think some people do put down hay or sand in their runs but you could get more opinions asking on the coop/run forum:
https://www.backyardchickens.com/f/9/coop-run-design-construction-maintenance
https://www.backyardchickens.com/f/9/coop-run-design-construction-maintenance