The idea is that it's more "expressive" than one person = one vote. And it's more manageable than pure direct-democracy. Look, you could fiddle with things and have "minimum" amounts so that people can't completely break a government feature if they don't like it. E.g. 100% of people voting to have 0 military spending. But, if that were to happen, is that not a hint to politicians that none of the voting populace wants military spending?
To me it's another way to give more power to the individuals, rather than leaving it solely in the grasp of politicians.