There are a few reasons why people do this.
- There is a bug which SOMETIMES affects the end of season reset for teams who join mid season. When this bug happens the team is reset to n+1 starting design (where n is the number of races in the previous season). So the team is overpowered by 3dp in CD strength, 1dp in weakness and 2dp in the remaining 6 attributes making a total of 16dp more than the rest of the league. So that they don't have an unfair advantage from this bug many managers will leave and re-join during the season break to reset the cars to the correct starting design.
- If you research your CD strength it increases research power by approx. 10%. Some teams employ multiple CDs and cycle them during the season to maximise their research. The downside to this is that instead of a single high attribute at the beginning of the following season (which is what some managers want) the design strengths are spread across a number of attributes. Leaving and re-joining allows the manager to reset the car so that only one attribute is maximised.
- I don't know if it's still the case, but some months ago I noticed that promoted teams suffered from a lower starting design than the established teams in the tier they were promoted to. Again, leaving and re-joining allowed the promoted teams to begin the season on a level playing field.
AFAIK these are the only reasons, if anyone knows anything different please tell.
