Really, anything to cover the soil is beneficial. I suspect that cardboard is less permeable than loose mulch and probably gives a slight improvement for water retention. That can probably cut both ways, since it will also slow/prevent infrequent rain from soaking in.  

 Sounds logical, plus i do think there's something about cardboard that stimulates insect and microbial activity.  And in my area, we rarely have too much rain for extended periods, so the cardboard could help retain soil moisture.  Just my initial thoughts. 

 Yes to everything herein. 
It indeed helps the microbiology. 

Worms love it, mycelium networks grow faster and the water retention is a big win.