Chain tensioner
 

Most bikes with internal HUBs do not utilize a chain tensioner. But I also noticed that some, especially the more modern/sophisticated ones, do. I'm specifically referring to Shimano SG-S7001-8. So the question is this: why do some bikes with internal HUBs use/require (optionally?) chain tensioners. Thanks.

Because some other drop out duty is at play, like disk brakes, odd aesthetics or the anti rotation/reaction force washers/arms don't play nice with a drop out design which has an axle slot? Andy

I don't know about other duties, but disc breaks on internal HUBs can, generally, be removed if V-breaks are utilized without any additional adjustment necessary.

What? You ask why some IGHs use tensioners then say that if the specs are changed things are different. Duh. Sorry I don't follow your path here. Andy

I was referring specifically to disc breaks.

Internally-geared hub (IGH) bikes, like single-speed and fixed-gear bikes, obtain correct chain tension by moving the rear axle forward or backward in the long axis of the drivetrain. This is accomplished most efficiently if the frame has horizontal dropout slots. I think what Andy means is that if the dropout has other functions that require the slot to be more or less vertically-oriented, then chain tension can't be adjusted by wheel position. This is where the chain tensioner comes in, permitting the chain to be optimally tensioned when fore-aft wheel position change is not possible.
I hope this helps.
Please: "brakes", not "breaks". ;)

Steve has it right. My around the town bike has a SA IGH with a disk brake. It uses a Paragon drop out with a vertical slot and der hanger for future possibilities and the currently needed tensioner. Admittedly there is a small amount more chain noise but I get the ability to change out to a der system if I want. Andy

+1. If your disc breaks, you'll have a serious problem.

Or, to beat it completely into the ground:
:D