Every user account in Linux belongs to a mandatory primary group and an optional secondary group(s). Why do we need the distinction ?

When a user account is created using either of the commands: adduser or useradd, the default is to create the user account with a primary group that has the same name as the username.

Example
Create a user: jhon
sudo adduser jhon

Check the groups of the new user
groups jhon
output:
jhon: jhon

And the following will add the user to two secondary groups: group1 and group2
sudo usermod -aG group1,group2 jhon

Now the user belongs to the groups: jhon, group1 and group2

And, let us say, the user creates a file touch myfile.txt. What will be the group owner of this text file? The distinction between primary and secondary user groups serves to make the decision. And in this case, the primary group: jhon will be the group owner of the file.