why a 'data' container in the php+nginx example ?

I was curious why your nginx+php-fpm example uses a separate 'data' container based on busybox to provide the volumes. When I run the example via docker-compose the data container exits immediately, but the volumes work fine between nginx and the php-fpm container of course. This isn't a pattern I've seen documented before, is there a particular reason for the third tiny container ?