As I've come across Grovers algorithm I dont understand why when computing F(X), which is an oracle function people use classical reversible circuits(toffoli, fredkin) to evaluate the circuit. Why can't it just be done using classical logic gates(and, or, etc) when the function doesn't have any thing to do with changing the superposition of the bits.
Reversible gates are unitary, see for example https://en.wikipedia.org/wiki/Quantum_gate. The eigenvalues are $\pm 1$. The quantum computation model requires all gates to be unitary, so you can't just use classical gates.