It was a design choice to celebrate the material being used - concrete. I think only architects ever really loved them.
I live in a brutalist townhome and it exemplifies the things I love about the style, lack of ornamentation, function over form, yet scaled and appropriate to its location.
The simplicity and usefulness of the house lends it as much elegance as it needs.
It is not a facade, it is not cladding, it is the functional building its-very-self.
Brutalist buildings celebrate their exoskeletons.
It abandoned the far more ornamental Stalinist style for brutalism as part of Khrushchev's push for that.
Note the far more ostentatious buildings under Stalin and Stalin's rejection of Le Corbusiers extremely radical proposals for redevelopment of Moscow in favour of far more traditional designs.
To the extent it was later used as propaganda, that was a follow on effect once stagnation forced doubling down on construction that had initially been intended as relatively short term cheap housing.
Probably because it’s called good design yet it’s almost the cheapest possible design outwards. Convenient. But I think in 30 years we will facepalm at these white boxes we call houses.
Look at McMansions, which is a cohort of styles that are much easier to get wrong, and there are many objectively bad homes. But for minimalism, the worst you can often say is it's boring. Correct, but boring.