From what I see in the clips (and I started as a skeptic regarding this pick) Simpson brings
1. accuracy and a quick release (and all that goes into that, like footwork, making his drop quick)
2. ability to asnticipate openings
3. goes through reads quickly, sound decision-making
4. doesn't take off running prematurely - but briungs ability to (1) get out of trouble and make the throw, or (2) find open grass and head for it instead of taking sack
He's not a Lamar Jackson sort of dual threat, at least in my eyes. McVay could scheme for him, i.e. designed runs out of the bootlegor rollout, run right after faled jet left, etc., but he'd be primarily a pocket passer with enough arm to execute McVay's offense.