If you have a flip-flop atom then you can do some meaningful stuff; IIRC the gate-level circuit simulator that the Nand To Tetris course uses can't handle this

If you have a flip-flop atom then you can do some meaningful stuff; IIRC the gate-level circuit simulator that the Nand To Tetris course uses can't handle this stuff, and you can still build a CPU. I guess it just depends at what level you want to think about the circuit. Edit: maybe it should be called a "logic circuit" instead of a "digital circuit".