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: <code>adduser</code> or <code>useradd</code>, 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 <code>sudo adduser jhon</code>
Check the groups of the new user <code>groups jhon</code> output: <code>jhon: jhon</code>
And the following will add the user to two secondary groups: group1 and group2 <code>sudo usermod -aG group1,group2 jhon</code>
Now the user belongs to the groups: jhon, group1 and group2
And, let us say, the user creates a file <code>touch myfile.txt</code>. 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.

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. 

Linux Primary vs Secondary User Groups