I was looking at MH bulbs and saw this one, what could it be used for? It's 1,650,000 lumens.

Searchlights and those large moving spot-uplighters you see at theaters and business grand-openings. I actually once looked inside one (that was not operating of course), and it indeed had this lamp centered in a mirrored parabolic reflector, later that night also was also able to see it starting and warmup. I believe they previously used XBO short arc xenon for this application. 

They also have a use in large film/movie set stage lighting for simulating sunlight on very large sets.

I'm with bluehalide it's a stage and studio lamp, and GE has discontinued this lamp so they should be on a firesale soon, and you'll need 80 amps at 225 volts to run that lamp...

The newer ballasts for these are electronic, but still massive (for an electronic ballast), they're about the size of a 100A arc-welder. I once saw one at an electrical distributor, the fixture was smashed in shipping, and its remote ballast (which wasnt damaged) was put in the clearance section for the low low price of $4500.00!, it weighed maybe 30lbs and like I said was about the size of an arc-welder. It ran both 12Kw and 18Kw "HMI" lamps, I dont remember the brand, it was a specialty brand ide never heard of. 

I cant even imagine the size and weight the old magnetic ballasts were.

Actually these metal halide lamps are used for outdoor filming - the arc gap is too long for use in the narrow beam moving projectors.

They used to be used on outdoor film sets when clouds would otherwise reduce natural light levels too far and hold things up, leaving a lot of highly-paid people waiting around doing nothing.  In recent years films and digital cameras were improved in sensitivity and it reduced the need for these lamps, so indeed they do seem to be dying.  I used to make up to 12kW ratings but we stopped about 3 years ago because the demand was becoming too small.  Still have the ballasts though at work - the 12kW 80A ballast unit is well over a metre long about the size of a bathtub, mounted on some excellent quality large wheels!

Why that big ? ~40 2kW/400V chokes that would be connected in parallel dont get to this size. (assuming 400V 2ph supply)

Another Question: Is there anything preventing this lamp from being used in a specialized high bay fixture for general lighting?

For general lighting, this lamp have very short lifespan of few hundreds of hours, because of the very high loading, temperature and pressure inside the arctube.
The likelihood of lamp explosion is also much increased.

Still have the ballasts though at work - the 12kW 80A ballast unit is well over a metre long about the size of a bathtub, mounted on some excellent quality large wheels!

They sound like they'd make a nice room heater 

Why that big ? ~40 2kW/400V chokes that would be connected in parallel dont get to this size. (assuming 400V 2ph supply)

The lamp is worth about $4000.00 and they last 250 hours, so I'm just guessing here but I would say lamp voltage and current regulation are very important to squeeze that 250 hours out of the lamp so the ballast size really becomes irrelevant they just build them to maximize the lamp life and at a guess they are using an array of smaller switched mode power supplies whose outputs are all connected together which also would give the ballast a built in redundancy because if one power supply was to fail the others would carry the load, and good regulation offered by a switched mode also reduces the chance of a violent EOL failure, and switched modes do preform better on generators and can have much larger voltage input tolerances, and with large capacitors they can keep running even if the input power misses the odd cycle, the show must go on even if it's in remote areas....  

It was the magnetic ballast version at this size, so no switching power supplies or anything. An array of CWA's would not get to this size either

Also, i doubt that this lamp's life (however short) would be affected too badly by supply variations. Normally lamps in the high power ranges are not sensitive, and i doubt that the 250 hour limiting factor would be related too drastically to the current (as in, small change in current makes huge change in life)

With stage and studio lighting, noise is an issue when filming, so if they were to use passive cooling the ballast can get fairly large very quickly, even if they use active cooling for it to be quite they need larger fans spinning slower and insulation to reduce the noise, so that might also explain the sizing, but again it's only a guess I haven't seen the ballast in question, and at those power levels I can't see it really being a remote mounted ballast because it would need welding cables running to the lamp if it has to go any serious distance, not saying you can't do that but it's not really practical...

As soon as electronic HID ballasts became available, magnetic gear for such large portable lighting should've become obsolete. As I said above, I could pick up the ballast for this lamp with one arm and carry it like a small generator or welder. Lodge also described many other benefits of the electronic vs. magnetic for these expensive, short-life lamps. The projection spot uplighters I saw that used these lamps mustve had the electronic ballasts built into the backside of the fixture itself as I didnt notice any remote gear between the fixture and the generator running them.

They used to be used on outdoor film sets when clouds would otherwise reduce natural light levels too far and hold things up

They are even more important for filling the shadows under broad sunlight. A fill light of course needs to produce some lumens when used to balance against a main illumination of 100,000 lux.
And it does not appear that the big 12k and 18k lamps are obsolete in any way. Check out this presentation of related equipment: http://www.arri.com/lighting/daylight/

Photonicinduction has something similar... although they're linear and 12000W https://www.youtube.com/watch?v=UVntaa0DDhY&t=8s