Comment by pcwalton

It's important to remember the biggest reason why overcommit exists on Linux and macOS: fork(). When a process forks, the vast majority of the child process's memory is safely shared with the parent process, due to copy-on-write. But a strict accounting would say that the total memory usage of the system has doubled, which is too conservative in most cases. Since forking is so common on Unix, overcommit ends up being necessary pretty quickly. Otherwise, fork/exec would stop working in any process using more than half of the system memory.