The project needs to make bigger changes to a module. A guy starts doing the changes. It turns out that the task is bigger than we though originally. Team lead has a brilliant idea: you need help. So he'll assign couple of more guys to do the same change.

What's the catch? The catch is that we are now all changing the same files. The code is a mess and tweaks and hacks are needed all over the place. So basically one guy changes the files and others just watch YouTube and wait for him to commit. The it's your turn to change the files and the first guy watches PornHub.

You could all just try to edit the same files at the same time, but we all know how GIT feels about that. You change random lines, he changes random lines, someone else changes random lines, all merges go to shit, nothing works and we spend 2x more time on just trying to get it compiling again.