I have googled this so many times...

On shared systems, like Supercomputers, you often belong to many different Unix groups, and that membership allows you to access data from specific projects you are working on and you can share data with your collaborators.

If you set SGID on a folder, any folder of file created in that folder will automatically belong to the Unix group of that folder, and not your default group. You first set the right group on the folder, recursively so that older files will get the right permissions:

chown -R somegroup sharedfolder

Then you set the SGID so future files will automatically belong to somegroup:

chmod g+s sharedfolder

This is very useful for example in the /project filesystem at NERSC, you can set the SGID so that every file that is copied to the shared /project filesystem is accessible by other collaborators.

Related to this is also the default umask, most systems by default give "read" permission for the group, so setting SGID is enough, otherwise it is also necessary to configure umask properly.