Simulation is to test the functionality of the circuit design by Verilog, which is both the input signal and clock can be written in Verilog can be useful in checking the work of the circuit without creating real circuits.
Simulation is to test the performance of the circuit designed with Verilog which both input and clock signals can be written with Verilog useful in monitoring the operation of the circuit without building an actual circuit.
Simulation is that function test circuit design with Verilog both signal and input clock can write with Verilog useful in monitoring the work of the circuit without having to build the circuit