Fermentation tank question

[Shindig]

Why do some fermentation tanks come with a motor/agitator ? What is it used for? I have been on several distillery tours and never saw a ferm tank with one but I see them for sale here and there.

[Silk City Distillers]

Here's the sales literature from a company that provides a similar product. Thought it was interesting since it includes some quantitative figures about resulting improvements from mixing during fermentation.

http://www.alfalaval.com/campaigns/tankequipment/mixing-and-agitation/rotary-jet-mixers/Documents/Application_note_beer_fermentation_March_2010.pdf