Why do we need Primary (main) groups for users, what is the point?

Question

linux

Why does a user need to have primary groups what is the difference between primary and regular groups?

Answer 1

In POSIX systems, each file and each process must have an owner and a group. All basic access rights are tied to these UserID and GroupID.
That's actually when a user starts a process or creates a file / directory, for them this user is set as the owner, and the group of this user is set as the group.

Answer 2

When a user creates an object on the FS, he (the object) is automatically assigned the UID and GID of the owner. So, this GID is, in terms of your question, the main group. Usually its name is the same as the username, but variations are possible.
The remaining groups are technically no different and regulate access in the same way.