Does using reverse uphill in a FWD car give you better traction Why?

In theory, yes. The torque (turning force) is created at the front of the vehicle which is forward of the centre of gravity, therefore when you engage reverse the car will pitch forward due to the relative negative torque produced at the wheels. This weight transfer results in a larger force downwards on the wheel, thus increasing the contact surface area size of the wheel. The same would happen on flat road, it's just the situation is amplified uphill due to the additional weight transfer you get from including the gradient in the pitching moment calculations. Going forward in a RWD results in the same, due to the fact you've reversed the torque BUT you're now behind the centre of gravity.