Field editing, showing clients live renders as they give you suggestions for edits, being able to grab the ipad and work on some ideas while on the go.

All of these comments "why use ipad instead of Macbook to do x" has been answered a long time ago. The massive market sale through rate for the ipad ecosystem answers the justification better than anyone on this site can answer.

Same for the Lightroom/Photoshop on iPad folks - why use lightroom on ipad? so you can do it on the go without opening up your macbook. And use the pencil.