If there is no output port in code, Why the project will use 0 LEs after compilation?

Here is a simple case: assign B = A; assign C = B; Now assume that C doesn't get used anywhere in your design or hooked up to a pin, Quartus II will determine that there is no point keeping C. Now if A and B are not used anywhere else in your design then A and B will be removed since C is removed. If you are familiar with C code compilers the behavior is similar to the optimizations they do.