It's much faster (and smaller) to build such small images .vs building out the whole OS with Memcached installed (for example).
And you don't have to deal with OS configuration, etc.
Sometimes (okay, every day) I find myself trying a bunch of different things to get something to work. With Docker, you've got a series of "save points" along the way. You can continue building an image from the last good point, and end up with a really clean image that represents the shortest path from bare install to "what I need to get done."
I was playing around with a Perl script about a month ago that would strip all the RUN statements out of a Dockerfile, convert them into a shell script and use that to bootstrap a Vagrant box. So what I'm saying is, if you don't know what you're doing (or just like to experiment with OSes), Docker is really interesting.