Kevin
There is no performance advantage of leaving and re-joining.
As Titan Jr says it allows a manager to re-balance their start of season design if they do it during the break between seasons. It can also be a handy trick if you've had a low level CD or one with poor Strength / Weakness for most of the previous season and have managed to secure a better one during the break.
There is a longstanding bug where teams who join mid-season have a 16 design point advantage at the beginning of the following season. Currently the only way to remove this advantage is for the manager to leave, temporarily join an interim league and then re-join the original league.
Thank you Kevin, I believe it would have been because of the reason you mentioned, he was inactive for most of the season and had a bad CD and had got a new one, so just resetting the car. Just a few people in the league were a little sceptical, especially as he got podiums the first 2 races.
Thank you for making me aware of the mid-season bug! ?