What would be the use for an external tank in orbit?

It's an enormous, sealed, empty metal tank that they were already taking mostly to orbit already.

One would make a pretty serviceable space station.

100+ would make one hell of a space station if you designed them to be strung together.

Some early NASA concepts: http://www.astronautix.com/s/stsexternaltankstation.html

Cool! Did it contain anything hazardous while in regular use? I'm only seeing liquid H2 and O2. Maybe wall thickness would be a concern for habitation.

I'm not sure what was on the Shuttle tank, but other vehicles carry hydraulic fluid (for engine gimballing) & hydrazine (for small thrusters). They're usually vented at the end of a mission to safe the vehicle.

If you're wondering about practicality, Skylab was a Saturn V tank. Unlike the external tank suggestions, Skylab was built on the ground and never had fuel in it. But it was basically the same structure as the Saturn's third stage. Like ISS, Skylab had some micrometeorite shielding outside of the main tank.

https://en.wikipedia.org/wiki/Skylab

More recently, Nanoracks has looked at refurbishing ULA's Centaur upper stage into a habitat. Like the Shuttle, Centaur runs on a H2 + O2 cycle. Their plan is to lift an equipment kit onboard the Centaur. They'd cut a hole after the end of the mission and use on-orbit welding (not mature yet) to install all the floors/decks/airlocks.

https://nanoracks.com/in-space-outpost-demonstration/

No nastiness on the Shuttle tank; gimballing and RCS were left up to the Shuttle itself. It's basically just two tanks and some plumbing.

Thanks! I looked at some tank drawings, but couldn't tell if they were omitting auxiliary tanks for simplicity's sake. Didn't want to say anything inaccurate.