Difference between "usermod -aG" and "usermod -G" options in linux?

[Brief-Wishbone]

TL;DR

Difference between "usermod -aG" and "usermod -G" options in linux?

I'm on centos 7. I'm wondering what could be the differences between these two?
I've read this but could not grasp what they're trying to say. Can you give me an example scenario to work with?
https://unix.stackexchange.com/questions/608822/difference-between-usermod-ag-and-usermod-g-options

 

[pbuk]

$ id pbuk
uid=1001(pbuk) gid=1002(pbuk) groups=1002(pbuk)
$ # Adds project-a as a secondary group
$ usermod -aG project-a pbuk
$ id pbuk
uid=1001(pbuk) gid=1002(pbuk) groups=1002(pbuk),1003(project-a)
$ # Adds project-b as a secondary group
$ usermod -aG project-b pbuk
$ id pbuk
uid=1001(pbuk) gid=1002(pbuk) groups=1002(pbuk),1003(project-a),1004(project-b)
$ # Deletes all secondary groups then adds project-c as a secondary group
$ # (this is not often what you want to do)
$ usermod -G project-c pbuk
$ id pbuk
uid=1001(pbuk) gid=1002(pbuk) groups=1002(pbuk),1005(project-c)

 

[Brief-Wishbone]

thanks for the example.