I don't exactly know the current setup, but here's my guess based on what I've gleaned. There are 2 distinct machines serving as web servers for PM. They do not share any filesystems. There is another machine serving as a database server. Both web servers use a this server as a data backend.
Since no filesystems are shared between the two webservers, files (i.e. user node images) uploaded to one server are not automatically shared with the other server. This means that if you add a user image, it matters which of the two web servers you hit: you (and others) will only see the updated image from one of the two servers.
By using the database server to store the images, the images are automatically available to both web servers, since they already make routine connections to the database server for data. This means that either web server can retrieve *or* update the single image stored in the shared data store. Even if you have another web server serving just images, you still have to coordinate getting the uploaded images to the image serving server from whichever web server they are uploaded to.
The adopted solution is an elegant use of available resources. If implemented, a caching system could allay most concerns of waste (which will likely not be borne out in practice).