I often deploy web projects in a directory that's not owned by the user under which my webserver runs. Therefor, I often have to change permissions so the webserver can read from or write to certain files. So, for this example, let's assume I'm logged in to my linux box as brad, and I'm using apache which runs under the user www-data.
To give apache access to my public_html directory, I'd change ownership for the directory and all of its contents:
chown -R brad:www-data public_html
I may also need to allow apache to write to a certain directory:
chmod g+w public_html/somedir
Now, if I modify any files in public_html, I don't want to have to change the group permissions (that is, apache's permissions), so lets make the group ownership sticky:
chmod g+s public_html
Now, any new files that I add beneath the public_html directory will be part of the www-data group.