Basically it replaces your battery. You take your battery off, screw on the passthru, then plug the other end into a USB port. That way you don't discharge your normal battery while sitting in front of your computer for hours.
Some have batteries inline, others don't. I guess it's a matter of preference. I prefer the inline battery because you can unplug it from USB and it'll still power your e-cig for quite some time.
People have had varying luck with them, but I have been using two of them daily for a couple weeks with no issues.
Even aside from spending lots of time in front of a computer, they are also great if you spend lots of time on the road. Just get a cig lighter to USB adapter, plug in your passthru and vape on.
Heck, of you've got an AC outlet next to your favorite recliner, get yourself an AC (wall plug / 110v) to USB adapter (may need an extension cord) and vape all evening in your recliner without going through your batteries. (Okay, I haven't actually tried this last idea, but I
do have an AC to USB adapter. I might try this some time.)