In DSLRs, exposure bracketing can be used to achieve higher dynamic range in post, but what's the use of it in film cameras? Is it practical to blend 3 different exposures (negatives) in the darkroom?
It could also be used to have different exposure and thus a higher chance of getting it right (you can't see them on screen).
But film is also post-processable, though trickier and less flexible. Dodging, burning, even HDR were fairly common techniques in the film era. Just look at the history of HDR on Wikipedia.
As for why having the bracketing instead of doing it manually, it allows keeping (mostly) the same framing and avoiding subject motion, which would be much more significant in the case where you manually change the settings.
The use of exposure bracketing for HDR images has obscured it's original purpose, which was to ensure you got as single exposure that is as good as it can be for the scene. Camera light meters are not accurate and handheld units have their limitations. If you can't go back and want to make sure you got the exposure right in at least one shot you could use the bracketing feature.
This use case still applies to digital photography. In fact it's even more relevant as the price of film no longer applies. In every advanced discussion of metering (modes, techniques, expose-to-the-right, spot metering linked to AF point, RGB metering etc.) the obvious answer, at least to me is just bracket everything. Then you can pick the correct exposure afterwards, no matter what the camera's light meter may have decided, and delete the rest!
I couldn't imagine superimposing more than one negative onto a single print. even minor changes in the composition or small misalignment of negatives (film) in the enlarger would blur your print.
Carrying around thousands of images in the film era meant having a sizeable backpack and hours squinting in a dark room. "Bracketing" is merely a term for the the simple process of indecision. Not sure if you have the perfect exposure? No problem, take three shots at three different exposure settings. Adjusting speed was most common.
When you're printing your negatives, however, superimposing more than one is a great way to convince people in the 19th century that ghosts exist or people in the 1970s that saucers can fly.
In the days of film, you didn't have immediate feedback of if you got the histogram right or not (it might have been possible for some high end cameras to do it when they had 1000 pixel exposure meters - the 3d color matrix metering - on a film body - but that time was so quick in passing to practical DSLRs that it didn't make sense).
The key thing was, you didn't have the feedback. With print film, you often had a few stops of latitude that you could play with - black and white film had 10 stops you could work with (though I'll admit I might just be confusing that with the zone system). This wasn't the case with slide film where you had very little to work with. Using just one frame for a shot that you might not be able to go to again was such a gamble. So you'd always bracket it. With Velvia, I would often set my camera to underexpose by half a stop and then bracket +/- 1/3 of a stop on either side of that (Velvia's blacks held such detail in it that I could typically pull it out - compare to blowing out the highlights and getting nothing there -- though some people went the other way and shot it 1/3 a stop over exposed, just depends what you like).
And so, you bracket the shot to make sure that you got the scene properly exposed because you didn't know until you got it back from the lab to see if you've got something good. If the film captured the image better as underexposed or overexposed is something that you didn't know.
There was no use of the brackets for HDR for blending. It just didn't make sense. Might have been interesting to stack two slides together and expose the print with another stop in a darkroom... but I don't see that as all that practical there either (note: the Orton technqiue used an approach of two overexposed slides sandwiched - the use of multiple pieces of film sandwiched is a not uncommon technique - I just haven't heard of it used for attempting to get a greater dynamic range in slides). Instead, what people would do is either get the scene within the known latitude of the film (with a ND Grad filter) or work with dodging and burring the print as part of the darkroom process - give The Negative and The Print a read on that subject.