Idk how the USSR or it's republics did accounting, but even under socialist conditions it would be conceivably helpful to account for building construction on a per unit basis because it lets city planners evaluate the relative value of labor and resources being put into a building.
For example if you have two sites where you can build the same apartment block but one is flat ground and another is hilly or marshy ground, then it makes sense to calculate the amount of labor, additional resources, and specialized equipment you would need to level the slope or build extra foundations on the swamp.
Lots of companies and organizations also carry out internal accounting where they invisibly pretend to pay themselves and their own divisions for work because it allows management to see where money is being used and how much.
Probably because it's a gameplay abstraction? The alternatives are to either have a full supply chain of all things your people need to live and pay them with goods, which would be really complex and not very historical, OR you hire workers and pay them even if they're sitting around but then many players see that as a sort of timer and source of unpleasant stress.
Money is just a convenient abstraction for allocated resources in either case.