In my area, this isn't at all common.

I think the only justification is that some people like the space to feel more intimate while bathing.

Don't be surprised if you find ductwork or somesuch in there when you go to rip it out, though.

What oruboris said - it's definitely nowhere near "all" tubs, in fact I don't think I've EVER seen it IRL!

In addition to hiding ductwork, I suppose if you have high ceilings it might help keep the tub/shower alcove warmer to lower the ceiling somewhat.

I was going to get rid of mine in the lower bath and found it's full of duct work. MAJOR stuff (and normally I don't sneeze at moving major stuff, but this was too much to move). Plumbing corridor, electric and lots of venting. If you're lucky to find an empty one, there's no reason not to get rid of it. I had planned on it and the contractors had no issues with it until we punched in some holes and found what we found.

There isn't any ductwork in it. I already ripped one out and when I redo the main bath, I'm taking that one out too. I don't think I've seen a house around here that doesn't have this "feature". Just seems like there should be some sort of explanation as to why it's there if there isn't ductwork etc... in it. Would be more work for the original builder than just keeping the same ceiling height throughout the bathroom.

I think you answered your own question...everybody else is doing it. Stupid I know, but I'm sure the builder at the time was concerned about being the only guy not "doing it that way".

Remember that most houses built in subdivisions from the 50's on are built from stock plans. If they're 2 story homes, the baths are normally stacked on top of each other to save money on the plumbing. As such, those soffits (bulkheads) have a ton of junk in them from the bathroom above.

It Catches so more water from steam stays in the shower and wetting the .

If it is not to hide ductwork, electrical or plumbing for the upstairs bath, then there is no reason for it. It's truly a dumb idea. Makes it difficult to install even a recessed light over the tub because it would violate most electrical codes.

Yes, Mrs Pete hit the nail on the head. In a cold climate, the leaky windows of years past could make a small bathroom with its one heat duct considerably colder than the rest of the house. A lowered ceiling meant less space for the steam to fill. In my last house, we had the lowered ceiling covered in the shower material. In this house, we had it removed and still covered it with shower material so that the mold would not grow on the ceiling.

If nothing is in the bulkhead, then it was probably built to reduce expenses. Floor to bulkhead tiling would be much less expensive than floor to ceiling tiling. It is a way for the builder to reduce his cost. If there are 200 homes in a sub then that would be quite a savings.

Yes, it would be cheaper to tile to a bulkhead than to tile to a ceiling, but no tract house I have owned tiled to either. Just a six to seven foot height for tile that did not even reach the hole where the shower head came out of the wall.

The simple answer is that it's cheaper to frame/drywall/tape the extra bulk head than it is to tile it.

This is the builder saving on construction costs, not worrying about your comfort.

Except now, the extra framing may cost more than the extra tile!