The only reason I would carry a bivy is to keep out flying, crawling, biting bugs.
If these bugs are not a problem a tarp does the keeping dry, cool or warm part very well.
So then why not take a bug proof tent? ans. Doors are open to much and tent gets full of mosquitos and it is a full time job killing them. A tent has walls and is to hot in summer.

A tarp can keep the rain and sun off in summer and is cooler than a tent.