I understood this was done so the (cash-strapped) end user could reuse their own cassette without needing to buy a bulk loader and 30.5 (or even 15+) meters of film. The assumption seemed to be that if you shot 35mm, you had at least a crude darkroom available to load the film into a daylight tank and reload your cassette.
I’ve talked to someone that used to live in Ukraine during the 1980s/90s when they were growing up, and while this is kind of true, it’s also not. Here in the west cassettes were basically disposable packaging, but in the USSR they were all reusable, because stamping metal was relatively expensive. When you bought a roll with a cassette, you were buying the cassette too, which made it more expensive. If you have ever looked on eBay at old reusable Soviet cassettes, most of the time they actually have the price of the cassette stamped on them.
So, rather than being sold in a cassette, most film was sold as a refill that you could just plop into the cassette, and you could easily do it in a closet. Closer to the end, you didn’t even need to do it in the dark, there was an opaque piece of paper wrapped around the film that when you put it in the cassette and pulled out the paper it would give you the film, and all of this could be done in the light. Pretty ingenious solution if you ask me.