Rendering can be an extremely time consuming process. As we all know, time is money. From 3D Graphics animation packages to that final edit, Rendering (as necessary as it might be) is the official "Curse word" of the Film / Video Industries.
Project size, detail, complexity and length all have to be factored into the production pipeline process. "Render-Farming", is the process of clustering multiple machines to work as a collective to increase productivity, thus saving time & money.