The internal renderer can be two things:

• The batch, that lets you render several images in a row, in the meantime, you can still work on another scene or same scene in the viewports. 
• Or the hypervue network rendering for ProStudio, Infinite and xStream.

Rendering to disk uses the internal renderer, the old Vue 5 way.

These options don't need as many resources as rendering to screen, because it deosn't have to display the render in progress and keep the textures loaded in the ram. This is particularily useful when rendering hi res images and/or very large renders.

The downside is, you don't see what is rendering, so if it doesn't look like you thought it would, you will only know when it's finished.