Can anyone explain the purpose of this device? I don't understand if it's for a interface without a monitor control or what?

http://www.jblpro.com/www/products/recording-broadcast/monitor-controllers/nano-patch-plus#.V9xVu0srKUk 

I'll take a giant pass on that thing.
 
Bob Bone
 

It's a potentiometer in a box.
 
The most expensive part in it is probably the box.
 
It's purpose is to turn down or mute the volume and nothing more (except perhaps to make money). 
 
And I would say that it's an example of using very carefully worded but technically correct statements to get people to read something into it that they never actually say.

Suspicion confirmed

No one should purchase an audio interface that lacks a master volume but some people do. I think some of the Cakewalk/Edrol interfaces have an issue with sharing the master with the headphones as example.
And even if a powered monitor has a volume on the front,, your dealing with turning down both of them.
So those pour folks are faced with dishing out extra cash to add a feature that was missing. Therefore the existence of volume controls in a box.
Downside is it will certainly add another 4 connections and 2 more wires to the signal chain.
 
Sure you could build one with a project box and $10 worth of parts but that's true of a lot of gear.  
 

The TC Pilot(?) controller is nice and not that much more expensive, tho I believe they are out of production.  A friend bought one when his SSL was getting worked on.  It merely attenuates the full volume coming from you DA to powered speakers - you use it as a volume control.  Once he got the SSL back he liked it enough to put it in the B room.

It's a passive analog volume control. I have that exact one (older model that's blue instead of red), and use it to control the volume of the Mackie monitors that I have hooked up to my tv. It's a simple device, but does what it says on the tin.
 
The problem with having a master volume built into your interface is that they are rarely an analog passive solution. They're powered, and they usually color what goes through it depending on how high/low the volume is. They also have a tendency to be the first part to fail, which means you lose your entire interface for a repair. I use a Lynx card, and run it through a Big Knob because it doesn't have a master volume. Sure, there's one on the software, but I'd rather have a visible solution in front of me and keep everything in the box at unity gain. Between the Big Knob and my Allen and Heath mixer, I do all my monitoring outside the box, and don't have to deal with a lot of the stresses people often post about here on the forum.

When you use the software mixer to turn down the volume you actually reduce the bit depth of the audio.  That with the issues that ResSerp brought up and the fact that it limits routing options, makes a built in Master Volume knob a little less desirable than you might think at first glance.

Resonant Serpent
[...] use it to control the volume of the Mackie monitors that I have hooked up to my tv. [...]

One very good use for such a concept. A limiter to tame the volume of commercials is priceless.

The problem with having a master volume built into your interface is that they are rarely an analog passive solution. 
 
I can see that this if is true that we need to know this but then again it's only our monitor system which will be flawed buy a zillion factors anyways. I can think of lots of uses for the passive control,, but better or worse I doubt.