This is about thew new starter cost.

When a developer joins a team, they will not be as productive as they have to learn the code, frameworks, libraries, the project purpose, the tooling, etc… Often this impacts other members of the team lowering the entire teams productivity.

When you use productivity tracking (e.g. things like capacity planning) you will see the teams performance drop and it will take time for it to exceed the previous measured performance. This is the cost of adding a new starter.

So if it takes 6 weeks for a new starter to increase overall team producitivty then planning someone on a project for 4 weeks is pointless since the team will have a higher delivery rate without the extra person. This is typically why an organsation loses its ability to migrate staff between projects.

Code formating affects the layout of the code and our brains do all sorts of tricks around pattern recognition, so if your code formatting rules are too different a someone migrating between projects has to spend time looking for code and retraining their brain.

Its an additional barrier and a one within an organisations skills to remove (by forcing a common code standard).