Why fork() before setsid()

Question

Why fork() before setsid() to daemonize a process?

Basically, if I want to detach a process from its controlling terminal and make it a process

Answer 1

man 2 setsid, you will get the following description:

setsid() creates a new session if the calling process is not a process group leader. The calling process is the leader of the new session, the process group leader of the new process group, and has no controlling terminal. The process group ID and session ID of the calling process are set to the PID of the calling process. The calling process will be the only process in this new process group and in this new session.

If a process group leader is allowed to call setsid(), create a new session and a new process group(with the same process group id), it will lead to a process group id conflict.