A high-level language statement becomes machine instructions, which become voltage changes in transistors. Why is describing a computer at several levels of abstraction necessary rather than merely convenient?

D. Because no one can hold the whole system at once, so each level hides the last.

A line of Python may involve billions of transistor switchings, and the layered structure is what makes reasoning about that unnecessary. The independence runs both ways commercially, 1990s x86 programs run on today's chips because the instruction set is the contract. The abstractions leak, and knowing where is part of being good at this: cache behaviour, floating point, integer overflow.