You'll need to check the specific model's specs, but in general there are 2 types of passthroughs:
1) a "direct" passthrough, with no battery. A PT in the sense of the word, it passes 5v current directly through to the atty (or close to 5 v). Needs a 2.0 amp power supply to work well, since there is no battery to "boost" it. Draws a lot of current. Not recommended for computer connection; suggest using a USB power hub, charger, or "wall wort" with 2.0 amps available.
2) a "battery" with a built-in usb connection/charger. 3.x volt device with a battery that stays charged. Unplug and go kind of thing. Only requires about 400 ma (.4 amps) or so. Safer to use with a computer, won't draw excessive current through the port. (I don't use any of em off a computer, but that's me.)
You may want a "standard" atty for 5v... like the 3.0 ohm ones.
As to why - some days I only use a 5v direct PT and never mess with a battery, or charger. Reduces the # charge cycles on batteries too. Still, personal preference. They can be made for a few bucks if you like to tinker, or purchased inexpensively also, for around the cost of a battery (10 - 15 bucks).
Hope this helps. Good luck!